Well this depends on whether you're recording or playing live. The cabinet colors the tone of your guitar a certain way, so if you use your amp's cab instead of going direct into the PA and using the PA's speakers, it will sound different. I've seen a lot of bands where they run their amps to a mixer then back to the guitar cab, just so they can keep that tone. Some just use the PA speakers though, if they sound good for a guitar. I know Sumbersed uses the house speakers, at least they did at the Electric Factory in Philadelphia. For recording, I'd say definetely run through your guitar cab, don't worry about volume, just crank it high and mic the amp. You'll get a much better sound than if you run it through a PA. The difference between half and full stack, it's pretty much two cabs will be twice as loud. Maybe not exactly like that, but it's much louder, so you can have that higher volume without running your actual amp as high. Sorry for rambling, I probably didn't need to say half of that, but I hope I helped.