Well I’ve tried both, in the same camper, at the same time and I’ve got the opposite results to report.
The location was Black Rock Desert, Nevada in August, elevation about 4900 feet.
Temps around 100-ish.
I bring a 5k BTU window A/C as an emergency spare. I’ve tested it out and found it not quite adequate. It struggled to try to cool my camper, while the roof air had no problem even out there in the desert.
The other issue was it was at the rear of the camper and did almost nothing for the bed, where I really wanted to have chill space. The roof A/C mounted in the forward roof vent blasts cold air right where I want it, and the rest of the camper benefits from the central and high-as-possible location too.
In my application, 5k BTU and extreme-rear placement didn’t cut it.
It’s better than nothing, but that’s all it was better than.