Thanks for the suggestions! I'll have to look all those up. I went to see her today and she's 35 pounds and 39 inches. I really want her in a 5pt, I don't trust her in a booster yet (she's not quite 4- she turns 4 at the end of July so she would be a very young 4 when she starts riding with me). It might take her a while to gain the 5 pounds, she's not a big eater, so I might even be able to squeeze another year or so out of a harnessed one to 40 pounds. I really wish I could get another marathon (I love ds's!) but can't shell out the money for that.
I need the carseat by September.
The other idea I had was to buy one of the less expensive ones that harness to 40 pounds then turn into a booster. She can use that until she hits 40 pounds, then when she rides with me I could put her in DS's marathon and put DS in whatever I got for my sister, since he's smaller (29 pounds, 35.5 inches). That would keep her harnessed longer, which I would like.
Too many decisions

:
And slightly OT..... I went to a carseat check today and ds's carseat was perfect! Perfectly installed, perfectly used. Woo-hoo! I was shocked when the lady told me that I was only the *3rd* person to come in with a carseat installed correctly

They had already been open for 3 1/2 hours, and doing steady business (her guess was they'd already seen over 150 cars). There was a huge pile of carseats that they took to destroy, and of course replaced them all, which is good!