Well sales count and hardly anyone is going to buy a $350 handheld system. It's first couple of months sales will be that of hardcore Sony guys, rich kids and people who have to have all systems. Sony's current line up is Metal Gear Acid, which is a card game and a version of Final Fantasy 7: Advent Children which only works on the PSP, when it also has a DVD coming out.
While Nintendo might not be anywhere near it's power, it's more than half the PSP's estimated price, is exploring originality, employs wireless technology that encourages friend use (meaning kids will tell their friends to get it so they can chat to eachother e.t.c) it has a reasonable battery life plus a good amount of quality line up games, with Metroid Prime: Hunters not too far away.
There are rumours going around that the PSP will come in different packages, with the cheapest being the basic PSP, the more expensive having all the aadd ons. If that is true, it might still stand a chance.