if you want to decide what more important, then it's quite simple: do you want a phone or a portable gaming system more?
from what i know, there's not enough info on the phone to be sure what games it plays. it also depends on game developers themselves if a game is going to be supported on one or both.
you might not like what i think, but personally, even with the lack of info on the phone, i feel that ngp games don't directly translate to ericsson games. i have 3 main reasons for this:
1 - Software. the phone is running Android instead of XMB. (not that there's a direct translation of games vs. OS) unless the phone comes with an app to run ngp games or if it's built into their build of Android or anything else, who knows? that's a wild card. so it seems that the buttons are mainly for Android App games.
2 - Hardware the phone probably has the best gaming hardware for phones right now,, MAYBE. but there is one thing missing, the back touchpad. i read about a test game an ngp reviewer wrote and it pretty much described him pinching objects in the virtual world using the front touch screen and back touch pad. from what i know, the phone doesn't have any touch sensitive anything on it. (subject to change in the future though.)
3 - Development. The phone and the ngp are made by completely different departments of Sony. There's no denying that. at the unveiling of the ngp at a press conference or something, an interviewer asked a Sony PlayStation rep himself whether there was a tie-in with the phone. The Playstation rep seemed as if he didn't even know of the phones existance.
but this is my speculation, based on cold hard facts (that i'm unable to source sorry. ) but all of my speculation could be proven false in the future, so all we really have is waiting for both to be released.
(gonna call the xperia play xp for short.) Sony ia releasing playstation suite which is for android phones. Basically developers will release their games Through ps suite and it will just ensure a standard of games. Xp will be the first to have ps suite. It plays the same games that other android phones will be able to play, just making it more convenient with the buttons. I think they're gonna try to release psp1 games through ps suite. Ngp will have it's own games. The games on that are way too powerful to run on the xp (or any phone for that matter). It will be able to run games from the ps suite though. So NGP plays everything xp does but xp does not play all that NGP can play.