I have the Britax Marathon for my 16 month old, and I plan to buy the Britax Decathalon or Boulevard for my next child (due in a couple months). I have absolutely LOVED my Britax. It is well worth every penny! It's also easy to pull off the cover and wash (My daughter gets car sick so I do this a lot).
We went to Oklahoma to visit family a couple of months ago, and we rented a car seat from the car rental company. It was a Graco. There was nothing wrong with it per se, but it wasn't as user friendly (install or getting baby in and out of) as my Britax. I understand now why my seat costs so much more than the Graco at the store.
All seats have to pass the same federal safety standards, so they are all safe. It's really just a matter of how user friendly they are and how "pretty" they are (if you are into that type of thing like I am). Anyway, my point is, go with the Britax. You won't be sorry!