its a matter of preference. i have a quad and 3 thrusters and i prefer the truster set up. quads are fast down the line but i think they don't pivot as well as a thruster. i do like my quad tho on smallish (knee to waist) days that a very racy.
To me quads are basically toned down twin fins, they surf the same sorta way but hold in better and have more drive. Because of this i generally use them the same way i would use a tiwn fin, in smaller fun surf when i just want a different feeling board.
However many people find that they work great in big macking surf too (wardo rode a quad at teahupoo a few weeks ago), i just prefer the feel of a thruster more.
I'll surf a quad at a lax-steep beach break. Thruster comes out when the waves get hollow, fast and steep. I strongly reco a quad in the quiver. More speed. But you must surf it differently. Every board will surf a little diff. A quad is not a thruster is not a fish is not a egg is not a retro is not a bonzer is not a fun shape is not a longboard. You surf them all different. Quad is fun in the steeps but can slide out...but sometimes that will just get me deeper so its still fun. Whatever, buy one-it's just money.