i don't know that 2 or 4ohm will make a difference in the sound.
depends on the amp, again.
if it's class d, then it puts out the same power into 4 ohm or 2ohm
so no difference there.
but it will consume double the power at 2ohm, so unneccesary drain on the battery.
i'd say if it's a class AB amp, get 2 ohm
if it's class D get 4ohm
i have a 1ohm stable amp, so i got 2ohm and running them in parallel