Up until recently I would've rec'd the MA over the RA in a heartbeat b/c the RA is almost always outgrown by height before a child is old enough and mature enough to use a belt-positioning booster. I had two RA's for my now-6.5YO twin girls and while I loved that the seats were very easy to use and solidly built, they outgrew the RA's by height before they turned 4YO, while they can *still* fit a MA although they're just about at the height limit. I ended up buying a couple Graco CarGos on clearance b/c it gave them another couple inches of harness height to grow into (but still had only a 40lb harness weight limit and doesn't make that great of a booster), which cost more than the price difference between the RA and MA
But now there are some nice combination (Forward facing 5-pt harness/booster) seats with high height and weight limits, so buying an RA should be fine for most kids because then you can move them to a seat like the Graco Nautilus (65 lb harness weight limit, 19" top harness height, then converts to a good highback or lowback booster), or Britax Frontier (80 lb harness weight limit, 19" top harness height, also converts to good highback or lowback booster), which has the bonus of looking more like a booster seat while still allowing you to keep your LO in a 5pt harness until he/she is developmentally ready for using the seat as a booster. Although boosters *are* more convenient than 5-pt harnesses, be aware that there are significant safety advantages to a 5-pt harness (which is why race car drivers use them), and that before you move your child to a booster you want to be confident that the child understands the safety importance of remaining buckled and sitting properly in the seat without squirming around all over the place.
Here's a link that helps explain the safety benefits of a 5-pt harness:
http://www.kyledavidmiller.org/pages/4211/Car_Seat_Safety...