As people have said - it's down to individual choice.
I've never been keen on roof tents mainly due to the issue of striking camp if you need to drive to the shop for milk etc.
Having had various ground tents the downside is time to erect and pack if you are doing single night stops.
I moved over to an Oz Tent last year after seeing the demo at Eastnor and wish I'd done so years ago. The RV1 (smallest) will fit inside the 110 with one of the rear seats folded, the larger ones would need to go 'across' the rear load space with the seat folded.
I've also used a RV5 recently at Gaydon which had plenty of room for 4 and the height to stand up in. Both can be erected and packed in less than a minute and are completely weatherproof.
With the optional side and front panels you get masses of extra room.
Going to Gaydon with all the gear and 4 people in the 110 I carried the RV5 on roof bars and would use the roof rack for the 90 - no big deal.
If you want to be on the roof then the RV1 will sit nicely on the roof rack and pretend its a roof tent.