I have a 3.5MOA Burris,,, an RMR 7MOA, and even though the MRDS is sold as a 7MOA,,, comparing it to the RMR, it's a bit larger,,, I'd say it's about 9MOA.
I prefer the 9 of the Insight for the drills I do,,, but I'm mostly working inside of 15 yards. If you are going out to 25 often,,, I'd go with the 7MOA. The 3.5MOA is way too small for me to pick up quickly at any range,,, but it's also not as bright as the others. I have no experience with a larger dot than 9MOA,,, but from all the research I've done on them,,,, 7 - 9 MOA seems to be the size most often recommended for 5 - 25 yard stuff.