Here are some thoughts from someone who has had a hard shell RTT (James Baroud Space) on my daily driver for the past 120K+ miles and 300+ nights.
I enjoy the fast set/tear down time and ability to camp anywhere where my vehicle can go and I am not limited by dry or uneven ground. If the vehicle is off camber, I'll run over a small rock or dig a small hole. If I only had a ground tent, I would not have been able to camp on a prime lookout point at Mogollon Rim during Thanksgiving a few years back due to the intermittent slush / mud everywhere. With a traditional tent, you might need to compromise as the best place may be occupied, not be dry etc.
Hard shell RTTs typically run a little more than canvas ones- I chose hard shell because believe they are simpler to pack, can store sleeping bags etc inside, and I knew it would be a PITA to find someone to help install /remove 140+ lbs every time I wanted to go camping. The biggest CON though is being unable to park in many public parking garages. My vehicle + RTT is 8'3" and few parking garages can accommodate. There's also a mileage hit- about 1MPG in my case. Also, since not all National Parks/ places have dispersed camping, a RTT may not make such a big difference. In fact, Dixie Sods in W VA has better tent camping then RTT camping. I've since purchased a ground tent so as to not 'miss out.'
If you do decide to go RTT, consider the total height of the vehicle if you decide to leave it on full time. If not, consider where you will keep it when not in use. Keeping it on full time will allow spontaneous trips and open the door to some really special camping experiences, especially if you have a vehicle with the right mods.