Hi JB - hands off my red one! What don't you like about your XChallenge then...?
650 twin likes:
Fun, flickable, smooth linear power delivery, quick off the mark, good low down grunt, 220+ mile usable tank range (assuming you're not having too much fun

)
Sensible seat height option for anyone under 5'8" (800gs taller than 1200 @ 880mm) - tho' not an issue for you?
Narrower stand-over profile than 650 single - more comfortable when standing on pegs
650 twin "improvements"
DID chain, adventure-spec engine bars & sump guard, wunderlich vario screen, wilbers, 1200GS hand guards, wunderlich vario levers (small hand excuse)
650 twin "wish list":
F800 s/st power /top end but with GS ergonomics
Twin front disk 19" wheel
More adjustable suspension (shock & forks) as standard
650 twin issues
Early issue with Czech chains on 650 & 800 GS
Head bearings failing (at 12K & 15K on mine)
I had a series of warped front disks (starting at 800 miles) but sorted with a new front wheel & excellent BMW service from Rainbow
ABS sensitive to bumpy roads- kicks in too early - especially when you want the brakes to work
BMW: stop marketing the 650 twin as a beginners/girls only bike - it isn't! Just like the 650 single wasn't. Despite issues and wish list - it's not a bike to grow out of - great for touring Europe or B road playing. I wouldn't swap.