Unlimited w/J30
I grew up with the VW's - first one was a '68 with a center pop top which we used in Alaska, the Alcan, and all along the west coast. I've had three of them since that time, and I purchased a Sportsmobile which I would say is a VW Westy on steroids. Eventually, I've acquired a Jeep JK unlimited Sport (I also have a A-Liner a frame style pop up trailer), and I decided on the J30 top because, like you, I like to be able to "just go", or decide to "just stay" if I end up somewhere I'd like to stay a little longer while on a long drive.
There are a few downsides to the J30. One is it's not as easy to take the top off as it is for a rooftop tent, or just the stock top. It's not quite like having a Westy - as far as the interior goes - and you have to realize you will be cooking outside (not really a downside per se, but you have to be ready to cook in the rain/snow, or open the wallet for a restaurant in bad weather) - and you don't have the "couch" to sit at for meals, or relaxing at night. I have a 3" lift on mine, and a Yakima rack on top, and this limits the garages, or other low hanging places, I can go.
All that said above, I would not trade my J30 for anything...I keep a pair of down sleeping bags up in the top so I'm always ready to pull over for a nights sleep. I have slept in some low and high temps and have had no issues with either. The interior height, lighting setup, and the size of the window openings, all have made a great place to sit and read, stargaze, or watch a movie on the laptop/iPad at night. I usually get 16 - 19 mpg, and I would expect that is greater than a similar vehicle with a rooftop tent.
Here are some pics