Yes, I know that the correct answer is both. Maybe in a year or so I can afford to put an Aimpoint on both. Currently I have an H-1 Micro. Love it.
I have two SBRs. A 10.5" and a 12.5" For the time being I’m trying to decide which I should have as dedicated Iron sight and which I should put the Micro on. Please say and why.
Put it on the toy. (10.5)
Keep the fighting gun IRONS! 
If you had to grab one of those rifles for a gun fight which one would it be? Which one do you use the most for target shooting? I’m sure you have a favorite as to which rifle you enjoy over the other and that should be the one you should put you aimpoint on. But I would also put irons with the aimpoint just in case something happens to the optic. Best of luck!
I voted 10.5", but only because that would be the SBR that is my go-to gun.
If you 100% reliable, vetted, and dependable carbine is the 12.5", then go with that one.
Wouldn’t you want a gun that’s easier to shoot/aim be a fighting gun?
I would put it on whichever gun you will most likely use for “social purposes.”
For me that is my 10.5 LMT.
Just for those who want to know, which 12.5 upper and barrel do you have?
Right.