which would you prefer? SQ (good quality), SPL (good boom/loudness), or SQL (best of both worlds)

also, what is your ride? and your current HU?
for a limited budget, if your HU is good enough, just invest on a good set of separates for the front, mid-level sub and an amplifier.

it can get really complicated, but you can just stick to the basics. I suggest visit stores who can do demos, and check out which would suit your needs, but still fit your budget.