Here's the thing, you can like and see Piranha 3D or Scott Pilgrim vs. the World without hating other. These movies aren't mutually exculsive choices. As for what to see first, that just depends on your mood.
Piranha 3D is a movie for people that don't take themselves too seriously. I kind of wish that there were more movies like Piranha 3D being released more often in the theaters, if for no other reason then to have a nice break from all the preachy or dramatically pretenious self-important films that make up a lot of the regular releases. With Piranha 3D though, just a warning, there's a huge mass of gore and soft-core but full-on full frontal nudity. In terms of scale I'd say the movie is composed of 40% gore, 40% soft-core, and maybe 20% 1980s era creature shock moments. Dialogue is sort of minimal. In fact one supporting actresses, who's day-job is an exotic dancer, if I can recall correctly does not have any speaking lines.
Scott Pilgrim verses the World is pretty good though in the sense that it's a bit of a self-realisation moment for awkward youthful moments that endulge in a bit of escapism. However, by no means is Scott Pilgrim vs. the World a super deep or profound movie either; then again, I'll be the first to admit that I'm beyond the primary target age for the movie. It's all entertainment in the end, and both movie work for their purpose and your possible moods.