Not enough information yet. It may be a question of which do I buy first in the end.
If the price is the same, it boils down to soft support.
If both handhelds get the same big IP franchises I tend to play on consoles, it's a tougher call. I don't buy everything Nintendo publishes, so that's not the final word.
Hate to admit this, but two analog nubs is pretty huge for me. It means the same control set ups with analog stick for camera control as console, the lack of which has been my biggest complaint about the PSP. It's also my only major complaint about the 3DS.
If PS2 games are playable on the PSP2, there's no doubt I'll be getting one; it's just a question of when.