Logic (not the DAW!) says that the AC15 is the better choice because you can do number 10 on the volume knob and hear the valves sing, which I won't lie, is a great sound.
But, being honest, if you've got the space and soundproofing to use an AC30
, do that. It really does sound better. The bigger drivers have so much more powerful tone in them than the little one in the AC15, and even on 'Volume 7' the AC30
sings with a far more beautiful tone than the AC15 flat out.
Had a jam once with an AC30
turned up to about 7 or 8 so I could turn my Tubescreamer off because it was naturally clipping with a nice crunchy edge using a Gibson SG
with bridge pickup. It was the best guitar tone I've ever had.
As has been said, use a mic between the speakers. SM57
is the old fashion choice but personally I find I get a far nicer sound putting a U87
in between or something like that. People knock it but the TLM103 also does a great job there. Then I put a on-axis 57
on one of the speakers and an off-axis 57
on the other. (But again, if you have other mics, I do prefer the Heil PR20 over the SM57).