Hey wildjeeper here are some thoughts on the J30 vs. RTT. We owned a ARB Simpson III tent for 9 years and have logged a few trips with it to include a trip to the Arctic Ocean and the Labrador Highway. We switched over to a J30 about a year ago and have made a few short term camping trips so far. Each has advantages and disadvantages, for us we broke it down like this.
Storage - J30 (since it stays on the Jeep we have no need to find a place in the garage to store it and our 2 awnings)
Mounting - J30 (with it being always on the Jeep the days of carrying, lifting, mounting and bolting are over. Which makes it easier for those last minute trips)
Driving - J30 (with the lower profile of the J30 we have less road/wind noise, can maintain about 17 MPG, and have a lower center of gravity which help both on road and in off-camber situations on the trail. Also with the J30 I can still park in my garage)
Deployment - J30 (I think the others have covered this one but I will add that you have no need for a large parking spot. As long as you can park you have a place to sleep)
Sleeping - Simpson III (with the wife and 2 small dogs, we appreciated the larger sleeping area of the Simpson III. Getting up in the middle of the night is definitely a 2 person process in the J30. But one advantage the J30 has is that when it's 19 degrees outside you can run the remote start on your Jeep with the heater on to warm things up)
Usage - J30 (we really like the idea that we can stand up inside the Jeep and change out of our wet clothes (dry off the dogs) before climbing up into the J30.
Stowing - J30 (if need be the wife can easily handle that by herself in a rather short period of time)
Cleaning - J30 (easier to open, clean, dry and stow)
Weight - J30 (weighs about 260 pounds and the Gobi rack and Simpson III tent combo weighed in at about 293 pounds)
Height - J30 (has a height of 10" where as the Gobi rack (7") and Simpson III tent (15") combo is about 22")
Interaction - J30 (I have had to answer more question from both Jeep owners and non Jeep owners about the J30. Honestly John should issue you a pile of spec sheets to hand out when you buy a J30)