Which Car Seat???

I am going tomarrow to buy a convertible car seat. Please give me your opinions or experience. I am trying to decide between a few. The Britax Roundabout and The Britax Diplomat. Or The Graco ComfortSport. I have a VERY small car, the infant seat is a very tight fit right now. Even with the infant seat, I have to push the seats way up. My son will be rear facing for awhile yet, hes 8 months, but plan to keep him rear facing until I can. So which do you recommend??? Roundabout, Diplomat, or Comfortsport??? Whats the difference between the Roundabout and the Diplomat?? Are the pads removeable on the Diplomat?? Which would you recommend, considering my small car, rear facing, safety and comfort??? Thanks so much!!!
The Snugride is for infants, I need a convertable seat, Infant-Toddler Seat.

I have the Graco Snugride and its very good. it's one of the best rated carseats and its very inexpensive!