I looked at, and considered, a number of conversion vans before buying my SMB. You asked about differences. The conversion vans I looked at gave me the feel of something an executive might use for a mobile office (circa 1990's
), or a family might use for a road trip in comfort, but more in the way of "a big car," only with captains seats, the ability to sort of move around, and some fancy lighting instead of regular car seats.
The Sportsmobile is more like a miniature RV (although possibly without quite as many full systems). It has lots of cabinets, and less furniture (in general - of course layout dependent). It's more like something you take camping or adventuring (but can take on a road trip too, of course). There are more windows/screens. Possibly fewer forward facing seats, depending on layout. With a pop top, once you raise the top it's sort of like a magical tree house, and you really have two separate sleeping rooms (because it feels pretty private from upper to lower bed once you are lying down). On the other hand, when the top is down, there is not quite as much headroom as a fixed top conversion van or SMB.
My SMB, being a late-90's 2WD (With stripes! So not as cool/rugged looking) was pretty economical to purchase (and run). If you go 1997 or newer (in Ford) you get the modern engine with the ability to run Scangauge, etc. which I consider a plus (speaking of gas engines here). Also in 1997 Ford added the passenger side airbag.