I'm *no* authority on this, but our research involved printing out the consumer reports ratings, looking up the ratings/reviews in the "baby bargains" book, comparing the two lists, and poking at our favorite models in the store.
We aimed to put safety first, but we did weigh a few other factors (like weight of the carrier for the infant seat, I'm not a big person and had BAD carpal tunnel last time b/c of PE swelling, which resulted in tendinitis for months). Also, after renting a number of types of (crummy, cheap) convertible carseats from rental car companies over the past 3 or so years, we feel that unfortunately you do get what you pay for when it comes to ease of use in carseats. :(
For a convertible carseat (and I do not sell carseats, and with the disclaimer that it's very possible there are safer carseats than the ones we picked!!), we like our Britax Marathon. We find it easy to install properly, which seems to be one factor in safety, and the straps are easy to adjust to the correct tightness. Not being able to adjust the straps properly has been an issue for us with the rental car seats we've used -- we've had to literally uninstall and reinstall the carseat each time we got in the car in order to get the straps safely tightened. Where we live, Babies'R'Us stores have an actual seat from the backseat of a car there in the store, along with floor models of several seats, so you might go (or get DH to go) test out some models and see how comfortable you are with installation & strap adjustment.
We ended up getting the Chicco this time for the infant carseat, but can't give an opinion yet on ease of use (can let you know in a few weeks!). Last time we had the Graco infant seat -- it was OK, but I think our particular model got bad press for safety issues after we were done using it (that's what I seem to remember, I could be wrong) so we got rid of it before having another baby.
Hope that helps a little. Maybe others have more resources for researching safety!