convertible car seats. oh my....

Hi ladies. It has been awhile since I have been around. We are at the point where we need to get a convertible seat. we have the chicco keyfit 30 in our car right now. I drive a volvo s40 and I am worried about the "better/best" car seat recommendations in baby bargains fitting. I thought there was a website where you could see what seats will fit your car, but I can't find it now. Can you please share that website, or any recommendations you have for convertible seats?

Our son is in the 90%s for height and weight, so we need something that will keep him rear facing the longest but still fit in our small car. The Britax Marathon, Recaro proride, and first years true fit are what we were looking at but we have really, no idea.  Thanks!