I've got both.
I've HAD Rebel/XT, Rebel/XTi, then the 5Dmk2, then 7D.
My "plan" was always to go FF, so I specifically limited my EF-S lenses to the kit lens, and nothing else.
The 5D2 does it for me... could use better low light, but its WAY better than the 7D, or Rebels I had. I love available dark, but learned to shoot within the constraints of film, say to ISO 1200 or 1600 or so, but using fast lenses close to, or at wide open. Can't say if that works for you....
I ended up getting the 7D as a compliment to, not backup for, the 5D2. I shoot people, events... and the 5D2 is right there, works as I want it to, gives the quality I want. My other passion is birds... the 7D does it. It loses some quality, gets some reach, and has the speed and AF tracking that the 5D2 doesn't have. Different tools for different jobs. Both are value priced right now.