I have a Grandby and more recently purchased a near antique pop up trailer. Actually I’ve owned two, the one I have now and a smaller near antique trailer. They were purchases of opportunity more so than targeted ones. Years ago I even put the Grandby on a trailer a time or two so I could use my Expedition’s expanded seating for trips. As for pros and cons I offer the following in no order of priority:
1: There is more room/floor space for certain. It’s easier to sleep several grown people. Some models have basically a king bed or two facilitating N/S sleeping.
2: If you do more base camp/excursion type camping, IMO it’s handier to drop my trailer and set camp and not haul the camper everywhere.
3: Depending on what size pop up you are considering, the choice of tow vehicle expands.
4: For some, especially the mobility challenged, climbing into the trailer bunks is a lot easier than the overhead. Kinda is the same for getting in and out of the two units in general.
5: If you put stuff on the roof, it’s obviously easier with the trailer. That said they don’t take a whole bunch of roof weight, much like an FWC.
Generally storage between my trailer and my Grandby is a wash. An advantage specific to my trailer is the galley counter cranks straight up and down leaving the whole isle-way available for totes, bags and so on. Most trailers, the galley lays down or flips into the isle.
6: Putting away wet and drying out takes more effort than a FWC. Overall the Hypalon, or whatever material FWC/ATC uses for the soft side is IMO superior to the pop up trailers.
7: The flip over galley is a PITA IMO. Some have low counters.
8: If you’re an Overlander or tour type camper, It’s hard to beat an FWC.
9: Pop up trailers may be banned from camping in some bear intense areas. While I’m nowhere near as well traveled as many here, I’ve never been banned in a FWC although it’s technically soft sided.
10: I think it’s easier to heat a FWC easier than a pop up trailer, but not by much.
11: There is more set up with a trailer. Getting them level is more important as raising the top un-level messes with the lift system.
12: Most aren’t built as durable as what we’re used to. While I’ve pulled mine over Hart Mtn Refuge from the Catlow Valley side, The Steens Loop Road, All over USFS and BLM roads, with few exceptions they are not an off-road camper.
13: You never hear of rodents chewing on the soft sides on an FWC or ATC when stored. It’s one of the biggest fears pop up trailer owners have.
Good luck with your decision.