I have both in 18watt marshalls i built. Also the eminance wizard which is supposedly the same voicing. They are all good. The g12h is probably the most articulate, but the wizard is thick and chewy. The reaper i have in a unique 18watt type combo i built with 6v6's and a 6sj7 pentode preamp. So it has adifferent sound to compare with. But it has a nice fat tone to it in that amp. I had it in a el 84 amp and thought it was too bright.
I am so pleased with the Reaper 30 in my Rotten Johnny, as suggested by Paul Sanders. And the price is right. Broken in, it is an extremely well-balanced, all-around speaker...not over-emphasizing any one frequency like a mid-heavy Heritage Greenback, for example. 99db (spl). I have not compared it directly to the G12H30 70th Anniversary, sorry.
god i love my Reaper HP's (75 hz). so fat yet still bright enough. like he said above: well-balanced. but sorry can't compare to a real G12H, but beware there are a few different versions of the G12H as i understand.