My first suggestion is to project your son's height and weight using a growth chart. There's one online here that is exactly the same as the one the doctors use:
http://pediatrics.about.com/library/growth_charts/nboystw...
Just follow the weight and height limits of the car seats you're looking at until they hit the line of your son's usual percentile.
My son is in the 75 percentile for height and weight (that's above average), and probably won't be 80 pounds until he's almost NINE. High weight limits on car seats are usually garbage, in my opinion, because there's no way they're going to use it that long! My younger son is off the charts for height and weight, and he still won't hit 80 lbs until he's about 7 1/2. Do you really think your son will want to be in a five-point harness when he's 7? I'm assuming that's what Britax is bragging about when it says it goes all the way up to 80 lbs, because my Evenflo goes to 100 lbs as a belt-positioning booster. The harness accommodates up to 40 lbs. My son outgrew the height of the harness first.
My son has been in an Evenflo Chase (~$60) since he was 2. He grew out of the 5-point harness right before he turned 4. Let me tell you, it's a pain to do the seat belt instead of the harness--at least for us--because he used to buckle his harness all by himself. He can't reach the seat belt buckle by himself.
Here's something else to consider. We hand car seats down to our younger son. They are 2 years apart. He used the infant car seat, and is now in the convertible seat for a little bit longer (one more growth spurt, and he'll outgrow it). So we'll put the harness back in the booster seat and give it to him, and our now 5 year old will get a more grown up booster, like the Graco Turbo, or even a backless booster (depending on if our new car has high-backed seats). Since your kids are about 2 years apart, also, your older one probably won't be using the next seat you buy until he is done with car seats altogether. You'll pass it down to your younger one. That's the less expensive way to do it, because the grown-up boosters and backless boosters are much less expensive (~$20-30) than buying another big Britax!
The only thing that will really tell you the info you need is the instruction manual or info on the box.
Good luck with your decision. It's always such a confusing thing to research!