Thanks all. I'll take your words under advisement.
For the record, I'm months away from making this decision/purchase. But I like to be as informed as possible about my purchases, so I'm starting my research well in advance.
i dont think you will notice much of a difference, it seems my 5d AF is pretty similar to my parents 600D, although you might find the center point a bit snappier on the 5d moving up from the XS
That right there is fairly enlightening. To say the difference wouldn't be a lot means maybe I should rethink my plans.
Might I ask - why FF? Do you have EF-S lenses you'd need to sell? If your primary need is high ISO, the 5DII is hard to beat. But...if you're shooting mostly in decent light, have you considered the 7D?
Why FF? Well, I like to have fairly high-end things. If I had known the XS was the lowest of Canon's offerings, I would have picked up something a bit higher at the time.
Lenses. The only EF-S lenses I have are the kit 18-55 and a 55-250. I have gone out and picked up the 70-200 2.8L not too long ago, and I plan to grab a 24-105 in the near future.
ISO and 7D. Well, when the XS tops out at 1600 and most everything else is higher, anything would be an upgrade there.