THe main difference is how precise the lever travel is. SRAM has a name for the technology (Zero Loss), but basically the X7 have a lot of play in the lever before it start to shift. The X9 and XO don't. However, SRAM just announced they are bringing that tech to X7 as well, apparently starting now. So for the newest shifters the breakdown is like this:
X7 - zero loss, easy cable change, MMX (one clamp for shifter and brake) compatible
X9 - same as X7 minus easy cable change, plus ball bearings, lighter
X0 - same as X9 plus adjustable lever pull, multiple colors, lighterhttp://www.bikerumor.com/2013/05/23/sra ... erailleur/
Personally, I have older X7 shifters and I miss having the Zero Loss. If I was buying again, the newest X7 would be perfect.
I don't know how you will be sure you get the newest X7 though. You will have to do your research.