Over the last 6 years, despite having 3 children, we've been lucky enough to only actually buy a couple of car seats- we've been given 4 by various manufacturers to test (and keep). One of them has been a 5 point harness stage 2 seat, two have been stage 2 upwards with 5 point harnesses that have detached in order to use the safety belt and one of them was a shield based car seat.
In case you haven't come across a shield based seat, instead of a traditional harness that distributes the force of rapid deceleration across bone mass like the pelvis and shoulders, the shield based seats have a blocky wedge that sits on your childs tummy and wedges them into the seat.
Shield based car seats are on the rise in popularity as they're often cheaper and in some controlled testing they appear to be safer. I imagine that the wedge is intended to spread the impact over a wider area but I'm not convinced this better since stomachs are pretty soft and not suer awesome at absorbing impacts (ask your child to punch you in the stomach if you don't believe me). I have to admit we didn't get on with the shield at all, Fifi found it hot, restricting as she couldn't hold a book or her dolly in front of her and we dumped it after a couple of days, reverting to the stage 2 seat until she was big enough to use it without the shield but with the safety belt threddled through.
Britax are one of the four companies to have sent us a car seat, and when I had a phone call from them today asking if I had a few minutes to listen to their new car seat safety campaign I decided to have a listen. Personally I think it's worth a listen too but they can't call everyone of you up individually unfortunately. Fortunately though, you can visit their website and read it for yourself. Personally I think Danger's Versafix car seat that Britax provided for review is the best stage 2 seat going but to be honest, I think this transcends brand loyalty. Watch the roll crash video here and make your own mind up.
Disclaimer: I was not paid or enticed to make this post by Britax. Neither was any promise made of future reward. Reviewing the evidence, includied reading a lot of comment on http://www.car-seat.org and other independent sites. I can see that if practical, rear facing 5 point harness seats are 5 times safer than the best forward facing seat but these aren't always practical if you have 3 kids across the back like we do.