I would also go with the second upper and dedicated optic. I have no experience with detachable mounts, but even if they do as advertised, the zero is going to be different on different guns so you'll have to go through that again.
Rebel turned me on to the Sightron S-tac scopes. a 4-20X50 is less than $400 @ Optics Planet and they are very nice optics for the $$$. I must like them, I have them on four rifles.
The more I think on it, I am leaning more towards the second upper.
Never looked at Sightron before I've been wanting to try some of the Primary Arms offerings since they are about 30 minutes from me. After a failed hunting trip with a Millet not holding zero (the deer didn't even run I missed so bad) I've been using Vortex Vipers and scared to go back.
I also just ordered a Vortex Viper6-24X50. I've read so many reviews on them, I just had to try one. It's going on a Remington 700, either .308 or 22-250, haven't decided yet. If I don't like it, it will go on fleabay and I'll get another Sightron. At this time, my stable consists of 5 Sightrons, a Viper Red Dot & Magnifier on an AR, and the Viper on the way.