I think it depends on where your market is, and over what time frame you're looking at. In the U.S., it's pretty much entirely P1 since Sony pulled out. Overseas you may find Sonys in Japan, but I don't know if they have a different implementation all their own.
In the future you may find more PSI only implementations, since they are supposedly picking up more liscensees for Cobalt phones. However, I don't know what they plan in terms of modifying their own implementation. There's basically a lot of flux right now. If you wanted to, you could build a solution that supports both. But, the easier thing would be to develope for P1 in the short term, with the expectation that you'll probably want to (need to) change implementations in a year or two. It sounds bad, and perhaps it is, but it really shouldn't be too terrible. After all a contact will always have a name, and an address, even if the interface changes a bit...
For information on using the Palm Developer Forums, or to unsubscribe, please see http://www.palmos.com/dev/support/forums/