I'd say that if you got to the point of finding out which amp actually[i] is[/i] the loudest. . . . . you're using the wrong amp, and any serious headroom that you can achieve with either amp will probably be lost on a 112 cab. You'll need at least 2 112 cabs to reach a point where differences will become apparent.
I use a TH500 (and a 212) and even in loud bands I've never had both volume controls beyond 12 o'clock.
The decider for me would be the tone that each amp produce. They are both well built, well spec'ed and the Puma seems to be a modern, clean tone while the TH500 is more of a full, fat and vintage tone. As I said I chose the TH500.