We are pop-up truck camper users (for nearly 10 years now) who really investigated buying a hard-side at year 7, and then at year 9. We re-visited numerous hard-side campers during those periods, and were fairly turned off by the TERRIBLE lack of windows, truck suspension modifications needed for the additional 2200 LBS above our current payload, the vastly added maintenance on hard-sides that have much more real estate to maintain (inducing the many additional storage compartment doors to worry about rain infiltration), and INDEED the huge hit we would take on fuel economy (we currently get a healthy 13.6 MPG with our rig; gong with a hardside, we would be lucky to get 8 MPG !).
We simply like that we can park anywhere (literally anywhere) an unloaded pick-up can park (even in a good assortment of underground parking facilities), can go literally anywhere an unloaded pick-up can go at unimproved locales of all genres, can get the unit up our driveway (try that with a hardside rig on our property situation!), and many, many other benefits. We are MUCH narrower than virtually most hardside models (exception: a handful of narrow framed campers (but why get a hardside that is the same width as our pop-up anyway??); much, much, MUCH lighter; much shorter in the height dimension (by a LONG shot), and VERY roomy on the inside (nearly 7 feet clearance when roof is up on inside; no cabinets and walls and partitions on the inside making it feel like an inner-city rooming house)...
So, concluding, WE ARE long-time pop-up owners who considered (not once, but twice) very seriously buying a hard-side, and couldn't come close to justifying a move to a hard-side. Pop-ups (modern units) are not the canvas-sided spare utility boxes they once were in a by-gone era! They are today, luxury condos with wet-baths, basements and insulated side-walls.
When we get old and gray (some say we look that way now LOL), we will KEEP our pickup (and probably our pop-up as a unique guest abode on our property, after the $$$ rebuild I have just done on the camper), and move to a sleek and compact diesel B+ (the same footprint, perhaps 2 feet longer than our current rig: still smaller in width AND length than a behemoth hardside truck camper rig), and go tamer on the unimproved expeditions front.
Discussion: there was only one truck camper (a large hardside) that we have ever gloated over, because the rear of this camper had complete wrap-around windows at it's rear wall, with rear dinette/ AND kitchen), owned by the founders of Truck Camper Magazine. The only issue: it is no longer manufactured, and it would be way too large/heavy for our genre of use.
Good luck in deciding!