It depends on what you’re using it for. I’ve carried both and they both have good and bad points.
The Blackjack II:
1. Has 3G (fast) internet connection
2. Windows OS (which I personally find more difficult to use. Too many menus to dig through to get to what you want.)
3. Is compatible with Napster and obviously, Windows for syncing.
The Palm Centro:
1. Runs on the EDGE network (slower internet)
2. Is touchscreen (not necessarily good or bad, just different)
3. Uses Palm OS which is very intuitive (IMO) and simple to use
4. I had some issues syncing with Vista and Outlook, but was able to download a patch and it worked fine.
The best site for reviews is Phonescoop.