In very simple terms,
Spl is sound pressure level
Sq means sound quality.
A system that is rich in bass and along with loads of car rattling/flexing is an spl system. The main target of user building an SPL system is to make sure the neighbors/neighboring cars also listen to what you play in car. It is BASSY and LOUD.
An sq system rather focuses on the quality of sound of your system. The main target of user building SQ system is to make sure the cabin gets the quality of sound you may get in a theatre or hi quality surround speakers with equal and precise amounts of lows, mids and highs. Nothing over runs each other and you can feel the clarity being produced by the system feeling nice to ears. But always remember SQ system is for yourself not for the neighbors. A person in SPL (which includes 90% of our AWAAM) will never appreciate an SQ System. They'll always say bhai 'oonchi karo awaas' or 'bass ni nikal rahi... Maza ni hy system ka' and usually 'itna mehnga system...!!! tjhe kisi ne choona laga dia hy bhai'
SQ can be loud at times but a good SQ will never loose the quality of sound produced with punch in bass, clear vocals, and highs that are never annoying to the ears. SPL on the other hand is not very precise and all the three lows,mids & highs try to overrun each other. The sub enclosure is mostly sealed in an SQ system and vented/ported in an SPL.
In the end it all depends on the choice you make but mostly people start with SPL and end up being a SQ listener.
hope that helps. Experts can guide you better as I consider myself a speck of dust in the universe Audio Technicalities.