Obviously there is a lot of personal preference here.
Have you sat, or laid down in either?
For me, my first experience was an Air Force Cot. I am a tall guy, so I was longer than my Cot, I have broad shoulders, so I was also wider than my cot. Now since then I have gotten into several of the different tent cots that I have come across. I find them cramped, and still uncomfortable. I really think that I like the mattress in the RTT better. I also feel that I have more room inside the RTT. I have two kids, so, with the four of us the RTTs on the market include some very nice wide tents, which I speculate can sleep us all as well as our ground tent could.
Never forget, however, there is a very large "bling" factor to the RTT. It makes people oggle your rig, and ask lots of stupid questions.
I once saw a sign that said (to the traveler on this road) "Please put your vehicle into 4x4 now, and lock lockers. Tire deflation is recommended for better traction (No 4x4? That is ok, keep going, this sign is just to make those guys feel like they spent all that money for a reason)"
That is part of it with the RTT. People like to show off how much they spent and they like questions. Obviously this is far from the only reason people buy them, but it is a factor.
Next time you are in the area check out Cruiseroutfitter, he sells them, and I suspect he would be happy to give you a tour of the things.
Last thought. The RTT sets up on uneven terrain better, in my opinion, than would a cot. While digging may be required for a tire a time or two, to me it is not like the leveling needs a cot have.
Oh well, opinions are like breathing. Everyone has them, and they all stink at one time or another.