I put this in the GBE 1200/M9/SL thread too
My singer got shipped to Washington DC, so my band is unfortunately on hold for the moment - no chance to work with the Streamliner yet I do know that BuffaloBob4343's band plays a lot of hard rock and metal, and thus far his findings have been that the Streamliner sounds fantastic on his gigs. That said, given the way I EQ my M9 in the band setting and my findings of the Streamliner tone I think it will be great for metal. The low end is nice and fat, but backing the bass down just a little bit gives it a lot of punch... big, but still controlled. The SL doesn't feel as "immediate" as the M6/M9, but it's not sluggish either. I owned, practiced, and gigged an M6 for almost 2 years before picking up my M9 and it's a great amp indeed.
For me, it would really come down to tonal goals. If you think you'll want the sizzly, ultra-modern tone available to you, the M6 would be the way to go. Otherwise, the Streamliner really covers a lot of the same ground as the Carbines and does so with a fatter, warmer feel while allowing you to dial in some OD, which the Carbines do not do.