Okay, I'm going to make this a collective effort, since I have had an ML10 v2 on my hands for a very short time (I am returning it, as I purchased an ML310 Gen8 v2) and do not have an ML30, so am going on what I know from HP's QuickSpecs. I will try and make this reasonably authoritative, but I am sure Joe_Miner and SchoonDoggy can add some valid points. Note: Any approximate prices I mention are US; exchange rates may tip the scales in favor of one system or another if you're in another country.
So...HP has several tower servers that all seem to fit the same form factor.
The ML10 v2, at the budget end, has recently listed as low as $329.99 at NewEgg for the Xeon version, and who's to say it won't go $299.99 on sale at some point?
The ML310 Gen8 v2. This server is on its way out, being replaced by the ML30 (Gen 9) but is still available. The lowest price I've seen for the Xeon E3-1220 v3 configuration is $461 at ProVantage.com The ML10 v2 looks to be largely based on this with some features missing (more on that to come).
The ML30. The cheapest Xeon configuration I know of is currently around $649 at ProVantage (the HP SmartBuy configurations at this time here are sometimes cheaper than the base model, that also goes for the ML10 v2).
So what are the known differences?
The ML10 v2 and ML310e Gen8 v2 use a v3 Intel Xeon based on Intel's Haswell desktop architecture. This is still quite recent as processors go; we really didn't see any Broadwell (v4) Xeons in this segment, and Intel quickly went to the v5 (Skylake) architecture used in the ML30. The ML10 v2/ML310 Smart Buy come with a Xeon E3-1220 v3, base clock 3.1GHz, the ML30 Smart Buy with a Xeon E3 1230 v5, base clock 3.4GHz. If you're familiar with desktop processors, these are similar to a Core i5 of their generation: Quad Core (no hyperthreading) but with additional features that may or may not be available, such as Trusted Execution Technology, VT-d virtualization, support (in this case requirement of) ECC memory, and more.
Probably the most significant details of the change in processor are a modest increase in performance-per-clock cycle, moderate energy savings (although TDP is listed equally at 80w) and largest, that the v5 Xeon uses DDR4 memory and supports 64GB of RAM; the ML10 and ML310e with their v3 Xeons support DDR3 memory, and a maximum of 32GB. Memory prices are slightly higher for DDR4 right now, but not huge and this could easily change. If you are a tinkerer, note that used v3 processors are easier to come by and less expensive, so if you want the base CPU upgraded to a Xeon E3-1231v3 (quad core, with Hyperthreading, 3.4GHz base clock - basically the "Core i7" Xeon), which is one of HP's supported shipping chips on the ML310e (and should work fine with the ML10 v2), you'll pay about $200 used on eBay. The v5, you'll probably be buying new for a year or two until prices come down. As you can buy the ML10 v2 in a fairly base processor configuration, you may wish to do this, and upgrade immediately to save money on the initial spec.
The ML310e and ML30, being the higher up models, come with several notable features you may find useful. On the mainboard, you're going to find an internal USB port, and an SD card slot. Should you want to boot from a small flash device to run a hypervisor (example: VMWare ESXi, Citrix XenServer, or Microsoft Hyper-V) separate from your storage, this could be useful, but isn't a big deal if you're running one operating system.
I cannot confirm this for the ML30 (I think it does not and is an add-on kit, Joe can confirm), but my ML310e came with a front fan kit. This could be a big bonus for those of you running a hardware RAID card or high-speed hard drives. It could be a detriment if you value low noise. The ML30's front fan kit accessory comes with two fans, which could also boost the noise. The ML10 v2 is limited in this ability; its mainboard does not have all of the connectors. That's not to say you won't be able to make a modification to fit one; it just isn't likely to be one that will be tightly integrated so you can monitor fan speeds or control them with the system BIOS.
The ML310e and ML30 have a drive cage with a backplane and removable non hot-plug drive sleds; this can be upgraded to hot-plug by purchasing parts or buying a higher-end configuration if available. The ML10 v2 has a plain metal cage, no sleds, no backplane. The drives screw in to the cage itself, and the entire cage is unscrewed and removed from the system. This is far less convenient, but if your storage needs rarely change, you're saving money with the ML10 v2. While HP doesn't support it, several forum members are ordering ML310e or ML30 parts to see if they can change the ML10 v2 to match the ML310e/ML30 configurations. It appears likely, but doing so may raise the cost to the point where you were better off buying the ML310e in the first place. Also, the ML310e and ML30 have optional supported configurations supporting eight 2.5" drives instead of four 3.5" drives.
USB port configuration is different on all three servers. The ML30 has four front USB 3.0 ports, two rear USB 2.0 ports, and 2 USB 3.0 ports. The ML310e has four front USB 2.0 ports and two rear USB 3.0 ports. The ML10 v2 has two front USB 2.0 ports and 2 USB 3.0 ports. Also note: The ML10 v2 and the ML30 use Intel USB 3.0 ports; while I haven't checked yet, it's possible that as a Gen8, the ML310e uses Renesas (formerly NEC) USB3 ports. Those are probably the best after Intel, so I don't think that's a big deal, but your choice.
The ML310e has a dedicated NIC port for iLO, and is the only one to do so. All three servers have a dual-port Broadcom gigabit NIC, but the ML10 and ML30 have their iLO on a shared port configuration. The ML30 has an update to the iLO4 which may have or introduce additional features not available on the previous system
Because the ML10 v2 is not considered a "Gen8" or "Gen9" server (apparently it's in a class by itself), it does not support Intelligent Provisioning, which is the ability to do a low-touch operating system install right from the box. The ML310e supports this, but Gen8 servers only support the 1.x version of this, which currently stands at version 1.62b. The ML30 supports the new 2.x Intelligent Provisioning. At this time, I do not know what enhancements are available on the newer version, but if this is important to you, keep it in mind.
The ML310e and ML10 have an onboard B120i RAID controller, while the ML30 has a new B140i controller. The B140i lists as supporting RAID-5, but a caution; I don't think this is a caching RAID-5, and my experience with limited RAID-5 controllers like Dell's PERC H310 tells me this is a *bad* thing. If you are using RAID 0, 1, or 10, this isn't a big deal, but if you're using RAID-5, do yourself a favor and get a dedicated RAID card. Also note - As far as I can see, the ML10 does not list supported HP SmartArray configurations. That's not to say you can't run one, but HP may not consider it a supported config. I would probably run either a P222 or P420 just like those that are supported in the ML310e. The ML30 has newer controllers on its support list, however, these are considerably more expensive, and not as easily available on eBay for a deal. Note that running a supported controller has the advantage that HP's iLO can see the controller and provide you with additional information. While "not supported", I believe an aftermarket Avago-LSI controller like the 9261-8i or its successors will probably also run fine, albeit without the iLO reporting.
At this time, that is everything I can think of, but I'm sure more information will come out as more testing is done by Joe_Miner on the ML30. In the meantime, here are links to the HP QuickSpecs on all three servers.
HP ML10 v2:
HP ML310e Gen8 v2