First off, good call on the UHD's. I actually prefer them over Swarovski, the color palette is more true, whereas the Swaro seems to cast a green hue over everything to my eyes.
With that, the only person who can answer the question is you. Personally, I run 10x50's and you couldn't pry them from my hands. Brightness is off the charts, edge to edge is great, all the things. But the biggest thing for me how the fact that they're equally as useful handheld as they are from a tripod. I can one hand them while bowhunting, and they're plenty of magnification to get me out as far as I want to spot with binos on a tripod. A 12 is dimmer in low light, and gives a SMALL amount of magnification increase at the detriment to exit pupil and eyebox.
My dad runs the 12x50 UHD's and I run the 10x50's, so I've spent some time comparing them. I'm 1000% positive I've made the right choice.