12 ga's are dominant if you're in serious competition....but for most of us, it really doesn't make that much difference, especially on Trap singles from the 16 yard line.
I have a buddy that shoots nothing but 7/8 oz loads of 8's at 1150 fps in any 12ga ...for any game ...including Trap ...and his averages, as a casual shooter are around 96 out of 100. He's not a serious shooter ...he shoots 7/8 oz in his 12ga's ( like a typical 20ga load ) to reduce recoil and to save money on his reloads.
I've shot Trap singles with my 20ga Skeet gun (Citori XS Skeet model) - with a standard 20ga load ....7/8 oz at 1200 fps and a Mod choke ...and it breaks targets hard. I've also shot Trap singles with a 28ga Skeet gun ( Citori XS Skeet model )and my standard load of 3/4 oz of 8's at 1200 fps again with a Mod choke. My scores don't drop much at all, for Trap singles from the 16 yd line, with either the 20ga or 28ga vs the 12ga...but I'm not a serious Trap shooter either.
My primary load for a 12ga is 1oz of 8's at 1225 fps ...(I don't bother to load 3 or 4 different shells for different games)...and I shoot that load at Skeet, Trap and Sporting Clays - although I developed it primarily for Sporting Clays ...I may drop that standard load down to 7/8 oz as well to save some money on shot with a 12ga.
There have also been a lot of studies done on velocity and amount of shot in 12ga guns...and while there may not be a consensus yet....it does seem that 1 oz of shot seems to pattern better than some 1 1/8 oz loads...( less flyers outside the pattern) and guys talk about not getting their loads too fast - so they don't blow holes in their patterns. So more shot and faster ...are not necessarily a good thing.
I know some serious Trap shooters, when they shoot Handicapped singles ( 25 - 27 yd lines ) they do shoot 1 1/8 oz of 7 1/2's at 1200 fps...but their serious competitive shooters/shoot registered targets every weekend. If I practice with them ...I just stick with my 1 oz of 8's in a 12ga Trap gun ( Citori XT Trap with 32" barrels at around 10 lbs)...( and my aveages aren't great / but about a 23 from Handicap ranges - and a little higher at 16 yds) ...and that's good enough for just out having fun. If I were to shoot a 20ga at Handicap distances, I might give myself another miss or two as an acceptable round....but I'm not a serious Trap shooter.
Pay more attention to the weight and length and Fit of the gun ..especially if you want a dedicated Trap gun....than whether you will shoot 1 oz or 7/8 oz of shot....and it'll pay off way more in my opinion.
Trap is kind of the odd ball game in clay sports...because ideally we want to float the bird over the barrel - so we want a gun, that for point of impact shoots higher...say a 70%/30% pattern over point of impact or at least a 60%/40% pattern probably. Where in sporting clays and skeet we want more of a 50%/50% pattern ( and usually a lighter gun ) ...where in Trap we usually want a heavier gun ( in Trap there is less extreme left to right barrel movement ...and a heavier and longer gun helps with your sight plane and helps you with follow thru ) ....where that 32" or 34" barrel on Skeet with a 10lb gun, in my buddies words, feels like swinging a big ole sewer pipe, vs a fairly nimble and controlled swing of a typical Skeet or Sporting Clays gun.
While I have single barrel dedicated Trap guns...( BT-100, etc ) ...the Trap gun I like the best is a Citori XT Trap, 32" barrels, with adj comb, and with a GraCoil system in it and it weighs right at 10 lbs. I lust once in a while after a nice Krieghoff KX-6 in a 34" barrel ...but both of those guns are only available in 12ga...so your only option would be to drop down to a 7/8 oz shell...but the weight of the guns / and the GraCoil if you put one in....reduces recoil significantly anyway.
but I would still want the Citori XT Trap with 32" barrels for Continental Trap singles ...or Doubles. So I keep telling myself it would be foolish to shoot a different gun for Singles..../ which is why I don't shoot my BT-100 much either / so why spend the money on something I won't end up shooting too much....