i would say that it also depends on what you're watching the film on. if it's a smaller television, i think the comedy of hot fuzz plays out better than 300. if you have a larger projection/flat panel, the action/spectacle of 300 adds to the experience. now, i wouldn't base the decision SOLELY on this, but i do believe this does make a difference. since i assume you will get one now and wait to get the other later if/when it goes on sale, i would say get hot fuzz now, get 300 later.
as for comparing the quality of the films, yes, it's difficult because they are from different genres, but i would probably choose hot fuzz. it was funny, it is a movie you can leave on in the background, watch anytime. for me, i think 300 is a film you have to be in a certain mood to see.
still, i got both. when it came down it, i couldn't make the choice to get only one
but if price is a factor, i'd go with 300 now. it's $13.99 at several stores, whereas the lowest i've seen hot fuzz is around $16.99 unless u get that best buy 2/$20 deal. in a few months, both will probably be available for $9.99 or as part of their own 2/$20 deal, but to save money now, 300 would be cheaper.