If the seat is for a truck that needs a rf tether install or if you have space/width issues, then the MA would be a better choice. The MA also has more padding in the center and lockoffs for a seatbelt install.
This being said, I'd choose an MR hands down. I actually replaced my rf & ff MA/BV classics with MRs. I prefer the higher sides of the MR, the reinforcement of the plastic at the seatback when ff at the heighest harness height, the less brittle feeling plastic, more leg room rf and more leg support ff, and most of all the higher rf weight limit. My kids love the cupholders.
Although the fabric doesn't feel as 'plush' on the MR as the MA/BVs, I have found that the MR seat covers have held up just as well (if not better) than the Britax covers. The deep, plush fabric of the pale Britax covers (Olivia, Shannon, Madison) tended to absorb dirt deep into the fibers like a carpet would. It took quite a bit of effort to keep those seats looking fresh and clean. My Flair & Deco MR covers are suedecloth and have been no trouble to keep clean.:thumbsup: