I personally think the golf cart batteries are a better match for the service duty required by a big stereo. If internal resistance is a concern, a decent quality capacitor should have less internal resistance and a higher discharge rate than any battery. I'd be surprised if a cap made much difference at all with decent amps, it would be just fine without them.
Having said that, I like living with nice clean gelled lead acid batteries. No spills, the terminals never get fuzzy, and they have an extremely high power density (nice and small for their output but heavy). One thing about the gelled batteries is that they don't deal with over voltage very well. If you go this route, make sure you have a good quality charger that keeps the voltage below 13.75v.
So, my opinion is... there is more than one way to skin a cat. The right battery setup will depend on what you're looking for.
PS - Nice truck, MBrown.