I think your vision can play a big part in this decision.
My astigmatism can make a 3.25 moa dot on an RMR look quite a bit larger so if I start off with a dot that is already that size there are times where it just appears too large.
I did find that the 5 moa dot on an SRO worked better for me than the 2.5 moa since the dot was a little cleaner, but I also found that the auto brightness mode on the SRO just didn't work as well for me as the RMR so the bigger dot was almost necessary to see it.