We've had three different car seats for my DS:
Cosco Alpha-Omega 3-in-1 (aka Eddie Bauer): convertible seat that converts to a booster. (r/f 22lbs, f/f 40lbs, booster to 100 I think) Comfortable seat, but I had problems with the harness straps twisting. Also, it's commonly outgrown by height well before children reach the weight limits. My ds is in the 90th percentile for height and has almost reached the top harnessing position for this seat (remember with Cosco/Safety 1st 3-in-1's the very top (5th) position is ony for bpb mode, the 2nd to the top position is the highest position you can use for harnessing. Since ds is not three yet and needs to be harnessed for quite a while longer, I just bought him a Britax Boulevard recently to replace it. Just thought you should know. Some people really love these seats, but depending on how your child grows, it may not be the last car seat you'll ever need.
Cosco Touriva (now called the Scenera) approx $40-50 @
Walmart, Target, K-mart. Very basic seat, not a lot of padding (but ds doesn't seem to mind at all), nice high back, r/f's to 35lbs, f/f's to 40. Easy to install and use. A very good economical choice for those on a budget or as a secondary seat for the other car, grandma, or travel (which is what we use it for).
Britax Boulevard (formerly Wizard): Same shell size as the Marathon & Decathlon. 33lbs r/fing, 65lbs f/fing. Very HIGH top harness slots 18" compared to the A/O's 15", so it can be used much longer as a harnessed seat). Has "True Side Impact Protection" head wings, infant body pillow that is removable for toddlers, and knobs you turn to raise and lower the harness - you don't have to rethread it. This is a truly comfy seat. We bought the one w/ Park Avenue cover and it's really soft material. We're very happy with this one, but it was $$$.
Btw, Britax seats are well-liked because they exceed current safety standards, are designed to be easier to use. Other than the Sunshine Kids Radian seat (skjp.com), these are the only seats that can be tethered r/fing. After using Cosco seats for two years, I thought our Britax was a breeze to install. The only Britax seats that are NOT airline approved are their BPB's and also their youth seats Husky/Regent which are f/fing only and have 80lb weight limits.
One tip about installing a convertible rear-facing. Older infants don't need to be at a 45 degree angle like infants do. As a matter of fact it's actually safer for them to be sitting a little more upright once they have good head&neck control (helps reduce "ramping up" in the car seat during a collision). This will help the car seat fit a little better in the back seat. As a matter of fact if the infant seat fits, then the convertible should too.
This site has other recommendations for car seats w/pro's & con's:
Car Seat Recommendations and this site might also be helpful to you:
Car seat compatibility database Seat and Automobile Information.
hth!