So I took the plunge and splurged on a 50D as an upgrade from my Rebel XS, but now I am considering getting a full frame camera to go with it since I now have a nice lineup of lenses.
The question being, should i upgrade to the Classic 5D, or the 5D Mark II when the time comes to decide.
To be perfectly honest, the jump to any full frame camera would suffice in my eyes
You have just got your 50D. It's a great camera. My suggestion would be to have a total passionate love affair with the 50D, get to know it backwards and enjoy the output before putting too much attention on what your next upgrade will be. Quality time with the 50D is going to inform you in ways you could not anticipate. What is it about full frame that makes it a "must-have" for you?
I shoot full frame, 1.3x crop & 1.6x crop bodies on a daily basis and move freely between the three. My personal viewpoint isthat there is an overblown marketing department driven perception that full frame is something etherial and almost mystical...it's not. It's useful at times for sure, and very nice to have, but unless your kit is making money for you, upgraditis can be a financially punishing trap.
The magic is in the pictures you take.