Actually, the Paxton is the cheapest blower for the 4.0 OHV out there. That is if you can find it. They stopped production a few years ago. The Vortec and BBK are about the same price when it's all said and done, the Vortec might come out a bit cheaper.
The Vortec is a centrifugal supercharger in that it spins the air kind of like a turbo before it goes into the engine to make it faster. The BBK is an Eaton-Roots type blower which sits on top of the engine and uses interlocking turbines to ram the air down into the engine. I have the BBK kit on my truck. The BBK will make power across the RPM band, resulting in flat hp and torque curves (see my dyno thread) and good power down low as well as high. The Vortec makes more power up top, but not much down low. The BBK is also the more reliable of the two, and is easier to install. Both blowers can be made to run more boost, but it isn't recommended with the BBK, and the Vortec can run a little more than the initial kit without hurting the engine.
All in all, I recommend the BBK Instacharger kit. Read my 3 or 4 threads on it that should show up in that search query. However, be aware you might have to make modifications to other systems, and it's not a good idea to blow a high mileage engine.
Good luck!
