Great question and I look forward to the discussion. To my knowledge there are four ”Floorplans” of Rooftent on the market:
1) Wedge Hardshell. See Bush Company Alpha, Alucab Expedition 3. Typically these can only be mounted front to back on a car roof, and can only sleep two people.
2) Full pop top Hardshell - These are very similar to the category 1 tent, but they lift up at both ends. I only know of a few Fiberglass options; some would argue these shouldn’t be excluded from the above wedge design category since they are essentially an identical floor plan, they just lift at both ends. However, this also gives you more foot/head room, which some folks may need. Like the first category, these are typically only mounted front to back and can only sleep two adults. Again, typically no annex attached. On the other hand, the wedge design provides you with some handy interior storage on the slope of the roof (See the Expedition 3 interiors — they have plenty of places to put things and pockets in the roof panel) which is not available on the full pop top.
The advantages to both of the above are pretty similar — they tend to be super fast to put up and stow. They tend to be more durable than a comparable tent in the below categories. And with the aluminum offerings, they have robust mountings for things like awnings and ladders.
The disadvantages are that the floor plan is really limited. It’s basically a 2-adult tent no matter what; because they are a fixed length, you can typically only mount them lengthwise, and so they are limited by the width of the vehicle. Sure, you can have them overhang a bit — but too much and it looks pretty goofy, like a rig is wearing a wide-brimmed hat while driving down the road, and and if it sticks out too far it greatly increases the chances of hitting stuff. That brings us to the other designs:
3) Clamshell Fabric tent - These are the “typical” RTTs with a fabric cover. They fold out like a clam and support poles automatically lift to raise the fabric roof of the tent. They can be mounted either opening towards the back of a vehicle, or opening towards the side. This is preferable for some rigs — perhaps you have a 270 awning, so 3 of the 4 sides of your rig are unsuitable for a door; a tent that can be mounted to open sideways may be your only option. The other advantage is that you can get a variety of sizes, from two person lightweight roof tents to behemoth tents that can sleep 4. These also lend themselves well to annexes.
The biggest advantage to this category is the versatility, as I’ve spoken about. But they tend to be lighter weight materials and more prone to wearing out. They are also the slowest to setup and take down - there’s plenty of poles to put in, and putting it away means lots of tucking, zipping, and messing about on the top of a vehicle to get it stowed. Plus, there is a temptation to go cheap on a RTT like this, but you do tend to get what you pay for. However going for a better brand will cost enough money to make it worth looking at the next category....
4) Hybrid Hardshell Clamshell - The only one I know of that fits this category is the iKamper tent. Unlike the traditional roof tent, the iKamper Skycamp can sleep up to 4. These have all of the quick setup advantages of the hardshell wedge and pop tops, but they provide the floor plan versatility of the clamshells As well as the option for an awning.
My current tent is a category 3; my next will be a category 4.
I think that sums up the general designs — I’m sure there’s some I’ve forgotten though. If others have advantages or disadvantages to include, feel free!
altho i am clearly leaning towards a hardshell folder type (opening on passenger) but just trying to explore the pros and cons of what i may not even be aware of associated with the different styles.