If you liked the como I would get it. RF tethering isnt as important as ERF, rearfacing is SO safe already. If it will be in a truck where the seat may rebound into glass then I would push a RF tether, otherwise its just an added bonus. The como has such deep sidewings that I think IF it rebounds into anything she will be cocooned well.
It should save you money down the road to get a bigger seat than the MA... but if you dont mind saving up for another seat then you could still get a MA.
When looking at a seat you want to look at top slot height, weight limit for RF and FF, and shell height.
A seat is outgrown RFing when they hit the weight limit or hit the max RFing height limit(some seats its head even with top of shell, some its head 1" from top of shell). For forward facing the seat is outgrown when they hit the weight limit, their shoulders are over the top slots, or the tips of their ears are over the top of the shell.
Going by those guidelines the como is currently the longest last convertible, followed by the radian, and then the MA/DC/BV but I think the new triumph is competing with the MA there as well... its neck and neck for those 2 lol!
The stats on those 3 roughly are:
Britax- 65lbs, 17" top slots, 33lbs RFing, nice tall shell
Radian- 65lbs, 18" top slots, 33lbs RFing, nice tall shell
Como- 70lbs, 19" top slots, 35lbs RFing, nice tall shell
Now you really could get any of the 3, but if money is tight Id try for the one that will last the longest. I really did love my MA, and have never seen a radian or como in person so Im going purely by the reviews and photos that have been posted by others.
I really did love my MA though... I just hate that my own child outgrew it before I expected(hes a big kid though)
ETA: Of course I forgot to add, you need to make sure they will fit in your car properly before you decide lol! The radian is very long and sometimes doesnt fit well RFing in vehicles, and the como is pretty BIG it looks... so the MA might be your best option
and since you would use the radian for your oldest you could pass it down to her when they need to switch and then you could buy him a harness seat like the regent or
if he is ready by then, a booster seat.
Im sick, so please forgive my jumbledness