GEM-Tech verses AAC...
AAC makes some of the absolute BEST suppressors on this planet... with that said, Kevin is currently trying very hard to fill a LOT of military orders (so is Gem-Tech). If you send an email to AAC, Kevin himself usually returns your mail. He is rather short winded and not very patient (especially with complete newbies), but all in all he's a good guy.
My personal dealings with AAC have been both good and bad. I've always gotten answered what I needed answered, and I've NEVER been dealt wrong, but they have rubbed me in a way that agitated me. I perceived a couple of phone calls with them as them being..... not rude... but not real polite... But they do stand by their products, and most of the time to a fault. I've seen them replace items that should not have been replaced. I personally own a couple of items from them, and have been nothing BUT HAPPY!!! Good company, ok service, and EXECELENT-OUTSTANDING-FANTASTIC products!.
GEM-Tech... They make fantastic stuff, they're quick to get back with you (generally), when you call them, they take the time to answer your questions to your satisfaction, and they will not bull $hit you.
I've shot their Outback and Vortex-II cans side by side to my AAC Pilot, and to be honest their cans sounded quieter than AAC's (granted I have excellent pitch hearing... most can not hear the difference between them, but I can).
I have shot their Trinity next to the Evo-9 as well. Almost NO difference in sound. If you want a quality name, then get AAC's Evo. If you want a no-frills and reasonably priced can.. get the Trinity. The main difference between the two besides aesthetics is the Evo-9 has a reservoir that will hold a little bit of water (to shoot it wet) while the can is holstered and will maintain the water for about 10 to 12 shots. Where as the trinity will not hold the water in the can if you holster the weapon. (this is the reason that my old military unit(s) used the Evo-9s on their pistols, and the Trinity on the MP5’s). A 'wet' 9mm can, will almost completely diminish the ol' first-round-pop.
For all intents and purposes.... civilian usage... it comes down to what you want. Both manufacturers offer an OUTSTANDING product and both stand behind their work... to a fault. Both of their cans sound almost identical when compared next to each other (the untrained ear will not hear a difference), and both look stupid-cool on the end of a weapon! (unlike Knight's Armament... crap construction AND sound reduction, crap service, and UGLY appearances).
What is your budget? Its the classic argument of who makes better jeans? Levi's or Wrangler? THEY'RE THE EXACT SAME PRODUCT except you'll pay more for the Levi name. Same thing between AAC and Gem-T. Personally Im going to purchase a lot more Gem-T's products as they're cheaper and I dont give a rats-ass about promoting a brand.
(also Gem-T has been around a little longer than AAC has... they just have not spent as much on a marketing campaign as AAC has).