I'm obviously going to be biased in anything I say - so the best I can recommend is simply to look at as many reviews and opinions as possible on the two games, a good place to start is generally somewhere like 'Metacritic'
http://www.metacritic.com/
This is a site which collates all the various reviews (both online and offline) for games, movies etc.
PS> The FM PSP game isn't intended to be a port of the PC title and never will be, it IS a realistic and fun to play football management game - however porting the PC title (apart from being an impossibility

) wouldn't have worked imho because of the different in how people play games on a PSP to a PC (ie. more limited playing time, screen space, control system etc.).