It depends on what youre using it for. The treo 650 is far more advanced, has bluetooth, better camera, better screen, more professional, expandable etc.
But dont hate on the sidekick 2. The keyboard and form factor is WAY better than the treo. The phone is just as good. Its got basic PDA apps, but I found myself using it more for that than any other PDA ive ever owned cause it was so easy to use the keyboard. You can go from one app to another in a blink of an eye, type in, and close. Its extremely well designed. The keyboard on the treo was a pain to use, although better than no kb at all.
Plus the SK2 is like a third the price. Internet is slow and unusable on both of them, but IM on the SK rocks.