Best safety feature, in my experience, is the size and the strength of the vehicle the kids are strapped into.
We've had Britax seats since my two were born (one 3 yrs, one 6.5 yrs, however the newborn seat was never fitted to the 9000. The only comments would be to repeat the one about the passenger airbag, and the trouble we had with some vehicles where the rear seatbelt was not long enough. We currently have a Elite and an Eclipse that fit perfectly. My experience of buying is only that Mothercare and the bigger, well respected, department stores like John Lewis (which is where we got ours from)have any idea of how to fit a child seat, and will let you try them in your car before buying. THIS IS CRITICAL You must make sure that the back of the childseat is parallel with the cars' seat squab, i.e. contact is made all the way down the back. Also, if you've got leather seats, make sure that the back of the child seat is smooth, not with ridges, as this will leave marks.
My experience of car manufacturers accessory childseats are that they are from companies like Britax, sometimes not even with the logos or trim changes, and cost you twice as much!