Which version of GW should I get...Or does it matter?

I see three versions of gw and my question is is there a difference between them? Could some of the differences betwen each be listed? Thx in advance.

They're all totally different games. I would say start out with Nightfall if you have to choose one (Supposably that's the best one out at the moment).
Otherwise, I would just get the Epic collection so you can just get all 3.

I would start with prophecies. Pre-searing is a great introduction and many of the basic/most used/best skills are from the prophecies campaign. EDIT from your friendly mod - No recruiting

cannonfodder