My vote would be the 34. The MOS is good but a custom milled cut for the particular optic would be able to get the optic set into the slide deeper.
I think 9mm makes more sense for a target gun because of the sheer amount of choices in ammo out there. Ammo cost is much better as well.

 

The .45acp is inherently more accurate than any 9mm, and since you reload it shouldn't be an issue. However, you don't need and MOS gun to mount a dot, all of mine are milled for a dot. This is a G17 with a 25 year old 7 MOA Docter on a Lone Wolf milled slide. I used to shoot allot of .45acp thru my G21's, but as Danzig pointed out, bullets were less than half the cost for 125 gr 9mm than 230 gr .45acp, and the arthritis was not getting any better. I've also had two G34's over the years also, and if for just pure accuracy, the G21. So I am sure the G41 could be even tighter.

 

I shoot the G34 and the G41 both quite a bit, both with iron sights though. I’m more accurate with the 41. I like the 45ACP, and the bigger grip on the 41 just seems to fit me better. Good points have already been made about cost and availability of ammo.
As others have said, I prefer a slide milled for the specific red dot over the MOS setup. Here is my G17 with a Brownell slide milled for the RMR.