I went thru this same question 2 years ago when ds was going to outgrow his infant seat. We chose to go with the RA because of our situation. I was traveling just about every other month between Anchorage, AK and Seattle, WA, just ds and I, so every pound I had to haul thru the airport counted. Also, I had to install the seat in many different vehicles depending on who picked us up at the airport/etc. My family is tall (I'm the shortest of 5 kids and I'm almost 6' tall) so we always need the front seats back fairly far, not leaving much room in the back seat for a rearfacing seat. I don't regret getting the RA at all, but if I hadn't been in the situation I was in I probably would have gone with the MA because of the higher weight limits/taller shell.
For the new baby on the way we are passing the RA down and got ds a Regent, even though he's only 2, he's already quite tall and skinny, so I know he will get the full 6 years of use out of it. I'm trying to convince dh to let me get a Decathlon for the baby, but he's not buying it right now. Maybe by tax time next year he will let me get it, it just depends on how fast the baby grows out of the infant seat. Since the baby isn't due until October, we have a while to plan, lol!
So, that's the reason we went with the RA instead of the MA. I agree, if you have the money, go with the MA if it will fit properly in your vehicles.