Sorry can't give any advice on them as I have not used one, but the MOA thing means the 3.5 unit will have a 3.5 inch size dot on the target at 100 yards and the 7 a seven inch dot at 100 yards. It's size on the target will change with the range your shooting at:
200 yards the 3.5 dot will be 7" on the target while the Seven will be appear to be 14" on the target.
50 yards, the 3.5 unit will have a 1 3/4" inch dot and the seven a 3.5 inch dot.
25 yards the seven will have a 1 3/4" inch dot on the target and the 3.5's dot a little under an inch.
Etc.
As a rule people choose between these two based on if they want to pick up the big dot for fast close shooting or the smaller dot for a tad slower but more precise shooting. I did not know from your post if you knew this was the way they worked or not, so apologies if I wasted your time with it.
Regards, Blueduck