Ah, well a new baby is always a good reason to pass down a RA
.
The Marathon, like a lot of other seats, tends to get outgrown by height rather than weight first. My 46 inch tall 40 pound 6 year old still barely fits in ours, my 41 inch 4 year old is using the top slots, as she is just above the second to top slots. I think the second to top slots are the same as the Roundabout, so your ds wouldn't have a tremendous amount of growing room in it, but he would surely get a couple years in it, and you might find that at age 5 he'd be ready for a booster like the Bodyguard.
The MA is about as comfy as the RA, but it is a little different to use: the harness adjuster is a little different and some RA owners find it more difficult to get used to.
You'd get more growing room in the Husky, but it's not as good for sleep support. Still, it's very comfortable, and my kids never actually complain they can't sleep in it. You can't take in on an airplane (you can take the MA, though) so if you travel, that might be a consideration. You may also want to look at a booster, if he ever rides in a different car...the Husky is a bit annoying to have to move frequently.
Well, I broke into my full Husky Vs Marathon list there, lol.
If you really want to harness your kids for a long time to come, then the Husky is your best choice, that's for sure, and with our law being 6 or 60, it's not unusual for kids to be harnessed longer here than other states, so I don't think peer pressure should be an issue.
If you can find a store that sells both seats, maybe you could see if your son has a preference? I know MY boy couldn't care less about what seat I get him, but maybe yours will have a preference....