The internet on the sidekick looks better. The pages are resized so they look alot more like they would look on a computer. The sidekick data plan is also less expensive and comes with unlimited data and SMS. The sidekick has limited 3rd party apps, unless you are a developer.
The blackberry has very limited bluetooth. You can only use it to sync to your pc and for a handfree system or headset. You cannot do file transfer's via bluetooth. The internet pages load slower than a sidekicks and you will have to get use to using wap for a faster connection. The blackberry plan costs a lil bit more and with T-mobile it only includes 300 SMS. You can download more 3rd party apps.
Well thats a basic rundown on the 2, hope this helps.