I don't think the Miras would be considered an outlier of Rockport's sound or design philosophy, although it seems clear the company is now using different drivers in their new models that may very well sound different from legacy models. Not sure about the MAs in this regard. Whose "house" sound you prefer is anybody's guess, but aside from that you've got one speaker that's the cheapest model (now that the Mira Monitor is dead) from an ultra high-end speaker designer and another that's at or near the top of a very good but more mid-fi designer. Personally I like the trickle down situation more than trickle up, but again sonic preferences will be most important driver -- just another way of thinking about the choice between the two and what you're getting.
I'd also mention at this price level you've got a huge amount of excellent alternatives from the likes of Joseph Audio, Nola, Silverline, YG, Vandersteen, etc., etc. I'm sure the PL300s are good speakers, but I'd be sure to hear models from at least a few of these other manufacturers before making any decisions regardless of what you think of the Rockports. Best of luck.
I'd also mention at this price level you've got a huge amount of excellent alternatives from the likes of Joseph Audio, Nola, Silverline, YG, Vandersteen, etc., etc. I'm sure the PL300s are good speakers, but I'd be sure to hear models from at least a few of these other manufacturers before making any decisions regardless of what you think of the Rockports. Best of luck.