Want to buy a Britax Frontier 85, but have a question

[confusednewbie]

Hi all, I have a 3 1/2 year old around 30 lbs and 35" tall that is currently in a Britax Marathon. I want to give his seat to his 10 month old sister and get him a combo/booster. I went to Britax first because I love his Marathon so much, found the Frontier but it is so expensive and I'm thinking 85lbs is a long way away, not sure if it's worth the investment. Any opinions for or against this seat? Any other combo booster seats out there that are liked?
Thanks!
michelle

[KrisM]

It's a great seat. He'll use it for a few more years harnessed and then he can use it as a booster seat. Jools has a picture of her 13 year old still fiting in it as a booster. You'll be set with that!

Or, you'll pass it along to your younger one when the Marathon expires and buy a dedicated booster for the older one. That's what I've just done.