Question Diono Radian Car Seat? Frontier? Other?

I need help! My almost 2yo son is outgrowing his RF infant car seat (Graco Snugride). Originally, I had planned to move him into one of the two car seats (both Britax Boulevards) now used by his 5yo twin sisters, and to put the girls into booster seats. But one of my girls is thin for her age (35 pounds) and does not meet booster weight requirements -- plus after doing research, I'd rather keep both in a five-point as long as possible.

So I started looking at some of the Radians (120/RXT) for my girls, because they can convert to a booster later and are so narrow you can (supposedly) do three across in one row. I now have them three across in the second row of my 2009 Honda Odyssey and that configuration works really well for us, as it leaves my third row seat empty, so I can fold it down and haul all sorts of things.

But then I started reading threads saying the Radian booster is pretty useless, and I should get the Britax Frontier with more height room instead (also for some reason I don't understand, their booster is better?). But those are so huge I know I could not get them three across with a Boulevard. That is not a deal-killer all by itself, but reviews also say they are hard to take in and out of the car, while the Radians are easier, making my hesitation grow, especially if the Frontier boosters are similarly mostly a marketing gimmick? I think my brain is muddled from too much research, conflicting priorities, and the internet in general, where you can always find someone who loves/hates a product.

So should I go ahead and get the Radians for my girls and give my boy the boulevard and have everyone happy in one row and then just hope the radians will be ok as boosters when they outgrow the harness in a year or two/ pay for new boosters if they are not? I'm not sure how long the girls will be able to stay in the Radian in the harness mode.

Or are the Radian boosters so guaranteed crappy and the seat height of the radian short enough (18.5 vs 20 inches, I think I read) that buying them would be throwing money away and the best bet would be to get the taller frontiers and just give up the convenience of my all-in-one-row?

Or perhaps there is some other factor that will make either of these plans unworkable? Perhaps even the Radians would not fit three in a row with the Boulevard? I was planning to turn my son FF at 2, but now I am questioning that, and have no idea how ERF vs. FF of his seat may affect spacing issues as well. Aack!

Any advice would be most welcome. To me, safety and convenience are more important than $$$.

Daughter A - 5yo, 35 pounds, 44 inches tall
Daughter B - 5yo, 41 pounds - 45 inches tall
Son -- 23 months, not sure exact weight but approaching if not already hitting seat limits soon

The Radian booster is pretty much guaranteed to be pointless for you. Most boosters have a belt guide that moves up and down with the headrest. The Radian guides aren't really taller than the harness and it has a 50lb minimum to use it that way. Luckily boosters are cheaper and combination seats (forward facing harness to booster) tend to work well in both modes. Some narrower choices for combo seats like the Frontier are the Evenflo Maestro (50lb max, ~18" harness, not tall booster but cheaper); Evenflo SecureKid (65lb, ~18", taller booster); Harmony Defender (65lb, ~18", taller booster). The Frontier 90 is 20.5". Since your kids aren't exceptionally tall, they don't need the extra height but they are nice seats. I have no idea which combo seats will fit across the 2nd row with a Britax. The Frontier 90 is the easiest install ever. You might have read about the Frontier 85 that can be tricky to install. I hate to move the 85. The 90 is very different. Can you go to babies r us and try them to see what fits?

How much life is left in your girls' carseats and how well do they fit in them? Are they young 5s or older 5s? Also are you in Canada (ie can you get a booster rated down to 30 pounds where you live) Will the girls be angry to ride in 2 different seats? (booster and convertable)

The frontier's booster feature is not useless the way the radian's is, but it tends to not work well until the seat is outgrown by harness - which can take a long time since they have made the harness limits so high. And it is heavy and bulky which can be a pain as a lot of people really like something light and mobile by booster age. (I have 2 fr85s and am still not sure I will ever use them as boosters)

The boulevards say they have 16.5 inches of shoulder room. smaller daughter is now at 14 inches, other at 16, but I have no idea how slow/fast they will grow. Smaller daughter has always been light for her age, other usually about average. They will be 5 1/2 next month.They probably would not mind being in different seats, but I would not be able to live with myself if one was in a booster and the other in a five point and then the one in a booster was hurt or worse in an accident while the other safe. Morbid I know but I would always blame myself for not keeping them equally "safe." I guess I could go to babiesrus to try frontier/others but I doubt they would have two of same model to try? I guess no harm in asking. Any other advice/suggestions?