I'm taking this question as theoretical, where it is not a choice between specific, existing platforms. Rather I'm considering the question as:
If you carry a 1911, would you rather carry it as is? If there was a magical way for it to shoot 15+1 rounds of 9mm just as accurately and reliably and with the same size and weight, would you rather have that?
If you already carry a 45 that holds 13 rounds, you still have only the two options. Either you are only allowed to carry 8+1 in 45ACP, or the gun can magically hold and shoot 15+1 rounds of 9mm.
If your Glock 19 could magically shoot 8+1 45ACP, would you take it?
Well, if this includes 9mm being lower in recoil, I would take the 9mm most of the time. No matter what the platform, the 9mm will have less recoil in addition to a lot more capacity, and I don't think 45ACP is that much better at what we're talking about. Also, I would really like a magical 1911 that defies science and packs 15 rounds of 9mm. That would be way cooler than a G19 that shoots only 8 rounds of 45ACP.
If we're going to get really fantastical and say that you can shoot either/or in a [insert any gun of your choice] with no recoil, then I could go with 8 rounds of recoilless 45ACP in something like a Walther PPS.
Of course, it would be even more impressive out of a little NAA revolver, but I would rather have this in a gun with a really good trigger and ergos. It would be worth a fortune!