SPL is sound pressure level, how loud it gets. SQ is sound quality, how accurately your system portrays how the material was originally intended to sound. One isn't necessarily better than the other, it's all in your preference. The good thing is that you don't always have to sacrifice one to get the other, unless you're an extremist.
I am new to car audio, but spl and sq apply to audio besides cars. I believe spl is the sensitivity. The higher the spl, the more efficient it is in producing higher amplitudes (volume). As for sq, I am not sure, but I believe it stands for sound quality. As for which is better, it depends soley upon the listener. Some people like their system loud while others like it accurate, etc. Personally I prefer SQ over SPL, but SQ is not something that is rated in the specs you see online. You have to go out and listen for yourself to judge SQ.
PS. If any of the above mentioned are completely inaccurate, just ignore it. This is just what I believe so far after 3 days worth of research on car audio. =)