Best PSP Game on PSN

I need a long, good PSP game that I can buy over PSN. I have 15 bucks left over to spend. However, I only have 1 GB of space on my memory stick. What is a good game, It can be a PSP game or a PS1 classic. I don't want MGS since I already have it on the PS1. Right now, I'm thinking of  Puzzle Quest or Symphony of the Night. Which is longer, and which do you recommend?

While both Puzzle Quest and Shymphony of the Night are fantastic games, Puzzle Quest is longer.  If you could get both, you should.

I can't resist recommending a Castlevania game. Symphony of the Night

As addictive as Puzzle Quest is, Symphony of the Night cannot be missed. 

Puzzle Quest is better suited on the DS or PC due to the control scheme, though the PSP version is probably adequate.
 
And SOTN is a classic Castlevania game for sure. I think it lacks the larger/epic boss fights in more modern Castlevanias but it's still fun. The lack of R2 and L2 on the PSP, and no widescreen option means its definitely not the best platform to play it on  though.
 
Doesn't that Castlevania X Chronicles for the PSP or whatever include it with Rondo of Blood? 

Dude, that Castlevania game is PACKED with content. Two games, plus the older Rondo of Blood. 
 
Also, you get to choose which tracks play where, if I remember correctly. It's a cool feature. 
 
It's a steal man. For ten bucks, you can't get any better on the PSP. 
 
EDIT- OOPS, you meant PSN. Nevermind then.