Hi A.,
My sons are like your's -- very tall, very quickly. I have been through a number of car seats. Some because they got too tall, some because their shoulders got too broad. They are now 5 and 3 1/2 and I had to buy new car seats this past summer. They kept sitting sideways in their seats because their shoulders didn't fit in. I thought about getting just the flat bottom part, but we travel a lot so they need to be able to sleep comfortably in them. I ended up with an Eddie Bauer high back, convertible seat. It reclines to 3 positions, has a 5-point harness as well as the seat belt adjusters up high on the seat and arm rests that go up so you can get them out of the way. There are other seats that look the same, but they don't have the shoulder room.
They both outgrew the infant carrier within a few months. Then, I had a seat that they quickly outgrew the back-height. I loved my Britax, but they outgrew the shoulder space. Then, I had a Graco convertible, nice high back, but they out grew the shoulder space. Now, we have an Eddie Bauer (I got it at Babies R Us, but Toys R Us has the same one). The lady at the store said she has people that either love it or hate it. I told her that it had more shoulder room than the others I had tried (and I tried a lot!), she said that a lot of people were looking for high backs with shoulder space, so she was glad to know. She said that also made sense as to why some people hate it. If the child doesn't need the space yet, they may move around if there isn't a pad to support them. If you only want to buy one, I'd try this one.
Good luck, hope it helps!!