To my knowledge, there's no data that shows X brand seat would protect a child better than Y brand seat (properly used of course).
We all have our biases, but all seats either pass or fail the same seat of criteria to be on the market, they aren't "ranked" by the government or any other agency. (Consumer Reports rankings are dubious at best, and have much to do with factors other than safety. They like inexpensive seats and take prices greatly into account when doing their overall scores.)
A biggie of the Big Britax convertibles in my mind, is that they last SOOOO much longer in both RF and FF harness mode. Thereby, keeping a child in the "safest mode" for longer, WITHOUT buying additional seats.
ie, a kid can RF for longer in a Big Britax than in a Graco ComfortSport. So, the kid still RF in his MA is safer than the kid who got too big to RF in his ComfortSport and is now FF. And, a FF 2.5 year old is safer in his MA than his buddy who got too tall and is now in a TurboBooster. Obviously, lots of parents suck it up and buy additional seats to keep their kids RF or harnessed longer. Then they look back and say, "huh, should have just bought the big seat in the first place".
Things like the rear facing tether and side impact wings of the Blvd, logically add safety, but no one actually tests a non-tethered Evenflo vs. a tethered Britax.
Additionally, a car seat is something I must contend with multiple times a day, every day, for YEARS. I want one that's simply not annoying. I'll trim corners in other areas to afford that.