We/I have 2 kids, 2 1/2 and 1 yo. Between our 3 vehicles, we have 5 Britax', 3 Marathon Classics-bought at Target for an average $150 on sale, normally $200 seats. 1 Britax Boulevard that we bought for our firstborn son, was a bit more at $240 new from Amazon and 1 Decathalon, which was give to us by a sister in law and the seat is just like a Marathon.
Why Britax and why pay more money?
The seats are convertible, they will handle the kids from birth till about 60 or 70 lbs so they will last a while.
The biggest reason for me to get them?, they are comfortable and with lots of padding. Go to a Babies R Us and go feel the butt and lower back of the car seats out there, then go and feel a Britax. The difference with many seats that look good but have no padding is the difference between sitting in a church pew and First Class in an airplane. If I were a kid, I'd cry too in some of those car seats with nothing but a piece of fabric and some thin foam between me and the plastic seat. My kids have no problems with longer road trips in their Britax'.
Safety. Britax do rate very highly in the crash tests, they are safe seats. They are heavier due to the robust construction.
Lastly, they are easy to install/take out and swap or leave out if needed. Their LATCH systems are awesome and if I can, I just use the seatbelts, it depends which is easier for me as every back seat is a little different.