A rooftop on popup is generally 13.5k btu. Most portables are not that big. Usually 8k to 10k.
I used a 8k portable in a hardside popup. It would not cool below outside temp in direct sun. With reflective bunk and window covers, it would cool 10 to 15 degrees below outside in direct sun. Add a shade tree and it did well.
I have an 8k BTU portable. On its own it is worthless for cooling the pup. Last summer I had a power issue and only had the portable running for one day. I was in full shade in TX weather during summer and it did not cool at all. This was with full reflectix in the windows and my awning out as well. I wasn't able to cool the camper back down until I got the rooftop unit running again.
Humidity makes a big difference.
If you're going to TX or VA, get rooftop. If you're in the drier states Steve mentions, you don't need your AC to work nearly as hard and can probably get by with less.
There's really no comparison. A rooftop unit, even the smallest, puts out 13,500 btu and in Florida,
on the beach, in July, when temps during the day were 90 and humidity over 90%, it got WAY too
cold in the p'up, that I had to turn it off several times. 2 of the days we were there, it rained all day
and night so we spent quite a bit more time indoors than we had planned. Had I used a portable,
that would've made for a miserable trip.
If portable units were most suitable for the task of efficiently cooling the interior of an RV they'd obviously have become the de facto standard. They haven't, roof mounted A/C units are the standard.
Depends on where you live. I do not have rooftop AC because I live in California which is unnecessary to have A/C at coastal areas even in the summer. But if I want to camp in the desert in the summer I would need AC but I usually do not go outside the coastal areas in the summer. If I do, then portable AC would be the best for me. I do not want to carry dead weight (AC) where it is not needed. I used to have rooftop A/C on my first 2 popups when I lived in the Southern states and A/C was absolute need there. I brought my 2nd popup in California and the AC was a dead weight because I actually never used the it.
as for you, you can keep portable A/C in garage for those times when you do not need it.
also if you looking for a portable AC, get the one with 15,000 or higher BTU!