That depends, what does "high definition gaming" mean? In TVs it means 1080p, except when it means 720p. Or 480p in some cases. Ask the developer what HD means, in some cases it's high resolution devices like the X, in others it's just better for a larger format, and still others there is no "standard definition" version and HD is just there to attract people who like the idea of "HD" - whatever it means. There is no real standard.
What games do you want to play? Search for them on the marketplace and check them out for the PSP. Which one is a better match for you will vary wildly if you're looking for a first person shooter vs. a puzzle game. I think it's safe to say a PSP is probably a better all-around gaming platform than the X, but I think the X is probably a better smartphone than the PSP. You can compromise and look at the new Sony "PSP phone" and see if that tickles your fancy, but again it's all based on what games you want to play and what games are available for the platform.