We struggled with exactly this question, and finally made the decision this fall and bought a pop-up. (The bad news is that we spent our disney savings on it, so our first trip to the World has been postponed). I will go on record as saying I despise tent camping, mostly because there is no where to store all your "stuff". All the organizing, packing, loading, unloading, setting up is a nightmare, and I hate sleeping on the ground,even with airmattresses. We rented a small Coleman for a few days a couple of years ago and knew then that we were ready to move up from tenting, but we wanted a model a little larger than the one we had rented. In order to do that we needed a larger vehicle, so a few months later we bought a '97 Pathfinder. A month after that, we bought a house, which meant we could not afford the pop-up we had just purchased a vehicle for, and it took 2 years for us to get back to the point financially where we could buy the trailer.(We hadn't planned to buy the house when we did, and had just decided to start looking a little so we would know what features we were looking for when we were ready, and a good deal just fell in our lap. I tell people that I bought a house by accident, however, it has increased in value by $35k in 2 years!)
I say rent a pop-up for a trip or two, so you will be able to compare the camping experience before you put out a pile of money, and then do some research on the various makes/models. We had our hearts set on a Coleman Santa Fe, but in the end bought a Damon Camplite with a slide out dinette.
We took the plunge because it was a great deal, and we knew that no matter what else we looked at we were always going to compare it to the Damon. I can't wait for spring!