I would strongly encourage you to do some research about car seat safety before you go and buy your next car seat. Most parents rush to turn their babies at a year of age - it's become something of a "rite of passage" in this country. However, many studies show that children who stay rear facing up to two years of age have roughly half the chance of getting a deadly injury in a severe crash. The idea that their legs, which might be up against the backseat in an older toddler, will break in a crash has also been proven false - it rarely happens. In Europe, toddlers are not turned forward facing until they are between 2 and 4 years old. The American Academy of Pediatrics has recently come out recommending that parents keep their toddlers rear facing to the limit of their seat. (http://tinyurl.com/AAPrecommendation)
As far as specific car seats recommendations, we are a Britax family, and we love them. Recaro is also a highly-rated brand, and so is the Sunshine Kids Radian line of seats. Most children will grow too tall for their seats before they ever grow out of the weight limit, so be sure to check the butt-to-shoulder height when you compare seats as well as the weight limits. The Roundabout 50 is one of the newer Britax seats. It retails for around $200, instead of the $280 for a Marathon, but it has the same shoulder height limit on it. It's weight limit is 50lb, instead of the 65lb on a Marathon, but again, most children grow out of a seat by height long before they reach the weight limit. You should be able to find online deals for the seats if you google "Britax discount" or something like that. I often see the Marathon for around $180, if you don't want a fancy print. We bought a classic Roundabout (which is smaller than the new Roundabout 50) for my oldest, and we love it. My second daughter has been using it for the past 2 years. Now she is ready to move out of it so we can put the baby into it. We still love it, and I'm going to be totally bummed when it expires next year! If you do buy a convertible carseat, make sure it will hold a larger child. Many of the widely- available seats only harness to 40lb and 40", which is around 3 or 4 years old - too young for most children to sit safely and still in a booster seat. If you buy a low-weight convertible, you will either have to buy another, high-weight seat or you will have to move your son into a booster before it is safe for him.
For older children - My oldest has a Britax Regent, which we love. We got it when she was 2 and we were moving her little sister into the Roundabout. It is a beast, and harnesses up to 80lb, but it's super comfortable for her, and very highly rated for safety. However, you should not put a child under 2 years old in it. If you can make your current seat work for another year, the Regent might be a good choice. Someone recommended the Britax Frontier for your son, which harnesses to 80 lb and then converts to a booster. That seat is also recommended for 2 years and up, so it would not work for your son right now.
Another thing to consider when looking at the claims that "this will be the last seat you buy" is the life of the seat. Most seats expire in 4-6 years. Even if they can hold an older child, they won't really be the last seat you'll buy, because they should be replaced before your son grows out of them. Some of the higher priced brands have a longer life on them. The Britax Frontier, for example, is good for 9 years. The manufacturer's website should have that information on it. Honestly, if you are going to have more kids, don't worry about trying to find "the last car seat." You will wind up moving your kids through the seats anyway. If you do not plan on more children, I would still recommend that you find a good quality convertible seat without worrying about boosters. You will likely need to replace his seat by the time he is ready for a booster anyway. In any case, you can find some good quality backless boosters very inexpensively. The IIHS rated some of the best/worst boosters here: http://tinyurl.com/highwaysafety
From my research, as a concerned mom (not a car seat expert), here is what I would recommend:
* Keep your son rear facing for at least the next year, or as close to it as you can. If you can make your current seat work, go for it. If not, look for a convertible seat with a high weight limit on it.
* Move him into a high-weight harness seat, or turn your convertible seat forward facing when he is at least 2, or reaches the rear facing weight limit of his seat.
* When he is ready (Google "5-Step Test for Booster Seat"), move him into a booster seat.
* When he is 4' 9", he will be tall enough to sit in an adult seat safely.
If you want to contact me via email with any specific questions, I might be able to answer them, or point you to some very good car seat safety forums. There is a lot of good information out there. My best recommendation is not to be in a rush. Take the time to do some homework before you buy your seat.
Best of luck,
S. L