The dm6 is my favorite year DM actually. It doesn't use an eye pipe and I love it for that sole reason. I'd rather not ever have to worry about a plastic eye pipe inside my marker. Sure they keep paint and crud off your eyes... but is that usually ever an issue in the first place? If your eyes are blocked by paint your likely not even gonna notice it because todays loader will still keep up just fine. It's nothing a nice swab cant clean in 2 seconds anyway between points.
I'm sure people will disagree, but I believe eye pipes are just a gimmick dye uses to bring in more cash. Because after awhile all the newer models DMs (Dm7- 14 actually) will need them replaced. Even if they don't crack or break they wear down after awhile. It's a constant income for dye for years to come on their older models. Which is smart on their part... They're making more money.
As for the differences between the 6 and the 12? Not much really. Internally they're pretty similar. DM6 doesn't have an eye pipe, but it utilizes a fuse bolt like every DM has ever had and operates in the same manor as all the others. It's a few ounces heavier too. Other than that they'll shoot very similarly.
I'd just count the rail out... it's based off the ion platform. Doesn't have an lpr like the DMs and works much differently. It's basically a fancy ion with a UL frame but is less "accesorizable" than an ion. Which is the whole point of owning an ion based marker imo.
All this being said, I love DMs and would shoot any of them... don't let the eye pipes scare you away. I just really like the dm6 for the lack of one. One less thing to worry about.