A certain poster seems to have staked out this thread as his personal hobby horse. :W Oh well, to each his own. Even a grain of sand can potentially become a pearl, eventually.
Buying a popup instead of renting a TT? No, I would not recommend that. Now, if you wanted to rent a popup for a few days as your first outing, it could be educational. A popup is low enough that you may be able to see overtop it while driving, and they provide a good way to start an initial checklist of things to do when setting up/breaking camp with an RV and a list of things you need to bring (which will have some different things on it than your old tenting list). But it is by no means mandatory to try a popup prior to a TT, especially if you already know you want hard sides.
Personally, my first RV was a popup. We took one trip with it, from OK to ID, and we knew we didn't want to camp that way so we sold it. My DW was particularly unhappy when she could hear the bear snuffling around outside the popup, in that WY NF CG (national forest campground). I got splinters from the plywood undersides of the bunk ends, and they wanted to sometimes go cockeyed and jam up. Wind whipping the canvas, hearing all the neighbors... but you know how that is already. A hardside was much more our cup of tea.
Depending on how many people will be in your rig, I'd suggest that you try to buy a little on the small side. I've moved from a 23' with slide to having towed TTs of 16'-17' overall length, and I like them. They are easier on the drive train, I don't feel the need for a WD hitch (but I make sure 10%-15% of TT weight rests on the ball), they're more maneuverable IMO, and I can fit into some small NF CGs that a larger rig could not (such as Amphitheater CG above Ouray CO, beautiful views there). Of course, if you have family along besides the two of you, a longer unit will be needed.
