Move to a Marathon or go to a booster for thin 4.5 ds

[Marmy]

I need advice on which seat to move my ds who is 37 lbs and about 40 inches. I like the idea of keeping him harnessed as long as possible. Is he too big to go into a Marathon? Should he go into a belt positioning booster? He is currently in a RA and has loved it. I also need to keep two seats right next to each other as he and his little sister (who at 23 months weighs as much) like to be right next to one another. She is also in a RA. So, I guess I will need a new seat for her also since she is going to bust the weight limit before him! Thanks

[Joolsplus2]

How about one Marathon for now, and maybe see if she slows down a little in her weight gain, and you can hand it down to her later?
When he's 40 pounds and 43 inches, or real close to that, I'd recommend the Bodyguard...it's a super-secure booster...more trouble than other boosters to buckle, but it feels almost as safe as a harnessed seat.
He'd fit fine in a marathon, by the way...top height limit is 49 inches. My dd is the same height, but 31 pounds, and still fits comfortably rearfacing in it. If she were forward facing, she'd need the top slots, but I think it would fit fine for a year and a half or 2 years, easily.
Julie CPS Tech and mom to 2 in seats
http://www.cpsafety.com/articles/RFAlbum/SarahMA.aspx