The 800 has the advantage of being about 40-50 lbs lighter than your 900 depending on the mods you have done... Overall power will be close to the same, the 800 has more snap, the 900 has a lot more torque. Handling on the 800 will be a lot better, especially if your 900 hasn't been tweaked very well to your riding preference. The 900 also has the disadvantage of having a lot more centrifugal force from the larger motor keeping it harder to maneuver. There will be more 'fun factor' with the 800 due to it's lighter yet similar handling characteristics, but the 900 will tractor through things better IMO (track length plays a role here).
I've got an 09 700 (piped) and an 05 900 and I love them both, they just have different strong points and weaknesses. A lot of people call the 900 a "Tank" and I have to agree, I LOVE how it mows through stuff and tractors around... there are a few times the added weight and brawn of the 900 can actually be an advantage. On the whole though, the newer RAW chassis takes less rider input to maneuver, there's no two ways about it.
Good luck:beer;
