Neither. Both are inferior to Android - ala Galaxy Tab
However, the playbook looks promising... it's BB App world is lacking a few thousands of apps to even compete with other smartphones/tablets
The PlayBook is more for business/hardcore users. The iPad is for people who just want to sit down and enjoy some media without being hassled by a ton of UI/hardware distractions. I bought the iPad and I LOVE it and use it everyday. Of course, no one has really gotten to play with RIM's PlayBook so it would be unfair to compare them right now. If I were you, I would wait for the 2G iPad (out sometime around April 2011) and then compare it with the PlayBook, which should be out by then. Just keep in mind you will need to actually spend some hands-on time with either one before making a decision, as you can't possibly experience the entire product through the internet.
What makes or breaks a tablet is the user interface, which the iPad has DOWN; RIM is just getting started and will probably need some time to work out all the kinks.