The alucab sounds like it would be a good fit for me plus one other if we were very comfy sleeping close (I snore)... The ikamper sounds like it would be a big thing that fits in the same area on the car top. The alu cab would be great for attaching an awning and a road shower to either side and my maxtrax to the top. It would have great durability and is a great brand. Ikamper has way more space and is a solid design (especially the v2). If I get into a rain storm or a cold weather camping situation, it seems like a bit more space would be awesome inside the tent. I don't know that I would want to go hang in the alu cab, the ikamper, maybe. A dedicated annex room that was covered? Yes. It sounds like Alu cab needs to be mounted to cross rails, mounting directly to the roof increases the risk of leaking and it sags in the middle which cannot be good for the tent or the roof. Ikamper can likely mount directly to the 100 series roof rails and is fairly easy on/off. I would likely need to modify the ladder and mattress on the alu cab, would need to modify the mattress only on the ikamper.
Someone convince me to go one way or the other!