turboboosters expire after 6 yrs.
lol.
basically, the nautilus has taller top slots by .25"-.5"
BUT... and this is a big BUT (hehe)
The frontier makes THE tallest booster (as tall or taller than the Monterey)
It is a freaking AWESOME one at that.
SO... the question is... what do you really want it for... if it is for the 5yr old to last until he's older and can use it as a booster or pass it to a sibling for harnessing, then it's PERFECT because it has a 9yr life, tall top slots, and goes super tall as a booster.
if it is for the 3yr old... well, I wouldn't get either the Nauti or the Frontier for him. I'd get another Radian and rearface that child till 33-35lbs,
I'm assuming it is for the 5yr old.
Really, unless he has a long torso and you need every viable .25" (like *COUGH* me for my giant 90% 4yr old who weighs more than your 5yr old and has outgrown the Triumph that your 3yr old is in... assuming it isn't the Advance), money is tight, and/or you can't get a good install, I would keep it.
Now I just don't like the frontier (I'm in love with my Nautilus), BUT... there are a lot of factors with that... starting with that my 7yr old has a Monterey that is new... by the time his sister outgrows the Nautilus as a booster, if he's not out of that Monterey, I'm putting him in the Nautilus as a backless and giving it to her.
He'd be like... 11 or 12 by then!
Basically, I bought the Nauti strictly for the harnessed portion. I may by her a girly booster or see what is out there for kicks, but the Nautilus makes a really great booster and I also have the Monterey that won't expire until she's TEN (and since my oldest was 4'9" AT 8yrs old... yeah... two weeks before his birthday... I was watching his ears to see if they peeked above the headrest ... they were level... I stopped peeking when he turned 8 and called it done, and she's on the higher percentiles... higher than her other brother is... I don't expect her to need a booster after she's ten... if she does, a backless would do), I'm good to go.
For you, I think this is a great seat because your oldest can use it... then the 3yr old, when the 5yr old goes back to the Monterey.
and then it should last the 3yr old until either he's outgrown boosters or he'd be beyond fine in a backless.