If you are a glamper and an ExPo snob get a RTT, otherwise get the tentcot.
I'm trying to find a RTT within budget. After reading the thread about horror stories and such in Fireside Chat, the girl refuses to sleep in a ground tent. We don't camp out as often as we would like (maybe 3-4 times a year) and cant justify the price of the RTT.
For the convenience, added time to get alert if animals/stupid idiots enter your campsite, additional space, ease of deployment, better storage space, the RTT is a no brainer. The only downsides for me are cost and MPG hit.
If you have a roofrack and already put stuff on there, don't worry about your mpg, it won't be noticeable (between the fairing and then the jerry cans positioned before the RTT, i don't think my MPG will go down after i put a RTT on).
As others have said it also boils down to how comfy you need to be when in your tent, my girl won't sleep on the floor, so its either an air mattress or thick foam. The space of the air mattress, double sleeping bags, tent, groundsheet, bedsheets, etc, etc takes up a lot of cargo space. Space that could be used for other things. If you have a fridge and take along a dog and carry spares you quickly realize that space is at a premium...
Just my ramblings...
-Sam