I've had people ride in the back with little to no problems. The first time I had someone in the back there was no room... but the people riding in the front have to realize how much room is up there. The guy in the back was being crushed, but the girl in the front (who's about 5'9 and was wearing 3" heels) had her legs straight out and wasn't touching the top of the floorboard. After moving up (and still having a ton of room in the front for her) everyone was comfortable.
I'm 6'2 and have my seat as back as far as it will go, but can move it up quite a bit and be comfortable. I think the biggest caveat is that all the back passengers probably should enter on the passenger side. The power drivers seat is SLOW moving. The passenger side is manual and flies up quickly. There is an oddity with the passenger side seat I've noticed too if it's in the forward position to let passengers in the back. When pushing the bottom of the seat back first before pushing the seatback back, it will go all the way back, smashing those in the rear. If you push the seatback back first, then push the bottom of the seat second, it goes back to the seat's original position.
Long story short, the Charger is going to be the most comfortable. Just not getting around that. The Challenger I think though has the most back seat room of any coupe I've seen and is pretty comfortable for others. If you really want a coupe and are going to use it on trips, you can't do better than a Challenger.