Re: Whats the difference?
Just know that there is no "safest seat." Some seats are easier to use, and may therefore be "safer" because you are more likely to use them correctly, but any correctly used seat is considered equally safe to any other correctly used seat. They all pass the same federal standards, and that's all we know about them.
That said, the KeyFit seats are among the easiest to use. I think they and the Graco Snugride 35 are nearly idiot-proof. Nearly. The Safety 1st Onboard 35 is also a very nice, very easy to use seat.