I have always been a PES fan but recently started taking an interest in FIFA 08 on the PSP in my opinion is the best football game out there.
--Gameplay--
FIFA 08 comes with a new range of game modes:

Kick-Off - Normal exhibition game where you can play with any club or country in any stadium in the world.
Football IQ - Allows you to answer all different sorts of questions in 4 different categories,Local Game, National Cup, International Championship and Friendly Match.

Interactive Leagues - I have not used this mode yet, but it allows you to play in an interactive league with other players using the PSP's wifi connection.
Game Modes - There are some great parts to the game mode. Here you can play a tournament, go into manager mode where you can buy and sell players, set training routines, and also play or simulate you matches. There is also a challenge mode (easy and hard) 61 challenges altogether. Other great new playing modes are the Juggling (you simply follow on screen instructions using the D pad) and Wall Attack (hit different targets on a wall to improve your score).

i play this game on world class as i find it very easy once you get to grips with it which is why it can get boring

--Fun factor--
With all the new additions to FIFA 08 this is by far the most fun football game on a PSP I have ever played. I currently have a team in manager mode, play exhibition games and regularly try to improve on my juggling and wall attack scores.

--Reliability--
reliable game never had a problem and always runs as smoothly as hoped.
--Game controls--
The game controls are great, Using the joystick controls the movement of the players and is easy to control the players when jogging or sprinting with them. Another addition that I love and always use is the quick tactics which can be used with the d-pad controls.
Graphics on this game are not great but will improve with newer versions

--Audio--
FIFA 08 comes with a crap range of background music and the commentary is what we have come to expect from Martin Tyler and Andy Gray.
--Instructions/Manual--
Yet again they manual for an EA sports game is fully comprehensive and covers everything you need it to. Although I always find it more fun just to play and work out all the controls for myself.