Why combos are better than stacks:
- No chance to mix up speaker and instrument cables.* Bad Things can happen when you do that.
- $$$. Most combos are cheaper than their head + cab equivalents.
- Only one load to carry and fit into your ride.
- No worry about that "ohms" thing.*
* N/A if you are use extension cabs.
Why stacks are better than combos:
- Visual factor. Let's face it: stacks just plain look better (bigger, cooler, more intimidating, etc.) than combos.
- Instead of breaking your back lugging a heavy combo, you can break a stack down into somewhat lighter loads.
- You can mix and match heads and cabs (within the usual electrical limits) to suit your needs and/or desires.
Raw power USED to be an advantage of stacks, but this has changed as technology has evolved. Now, you can get hundreds of watts in a lightweight combo, or a head the size of a shoe box. Genz Benz, Markbass, etc. have changed that playing field in a big way.