Car Seats: Preschooler, Target

Car Seat Dilemma W/a 1 and 3 Year Old!

Hello Ladies, I want your opinion...I have a 3 year old and an almost 1 year old (well he turns 1 next week) and I'm trying to decide what to do w/the car seats. My 3 year old weights I think last time like 37lbs and has a convertible car seat and I was debating giving that car seat to the 1 year old that weights about 25lbs because he is a lil bit heavy and uncomfortable for his baby seat. And buy him a booster seat or let him w/that seat and buy a convertible car seat for the 1 year old. But I'm wondering what's better...the age wise...