There are a lot of things to consider when deciding on which forward-facing car seat to get your kid/s. I solicited much advice from many of my friends who already had kids. Much of it heeds on brand and long-term comfort for the child and durability for the seat. Some of it is about the bulk - some of these monster car seats, especially if you have more than one kid, demand you purchase an SUV or minivan!
I had one friend who told me there's no reason to get a car seat that allows for more than 40 lbs because, by the time they are that age, their self-awareness will demand that they get to sit in a booster "big kids' seat" like their older friends. So, why bother if they're just going to whine until you give up and move them to a booster.
We bought the seats that work up to 65 lbs. mostly to stave off having to buy another seat for another 25 lbs. After watching this, though, I think that after the girls are 65 lbs., we just may buy another 5-point harness car seat and leave the booster for someone else's choice.
If you have kids under 80 lbs., you should watch this very emotional, but educational video one family made to prove a point about safety and car seats:
