I've said a few things in other threads about the Bissel. In short, it's an OK machine, and for the price, it does what you should expect to do. However, for professional use, it has no place other than sitting on the shelf for "emergency" situations where your actual extractor breaks down and you're stuck.
Mytee is a popular make for several reasons. They're very reasonably priced being the first and foremost. The parts they use in their models are also of high quality. Their pump is a SHURFLO with automatic back-pressure shut down (if you have the pump powered but aren't running solution through the line (spraying with the trigger), the pump shuts off instead of constantly running and burning itself out because of excess pressure build-up). Their connections are higher quality quick-couplers than you see on other extractors. Their heater is in-line vs an element inside the tank, so you don't have to wait for the entire solution tank to heat up. Their entire housing is a nice and sturdy, yet soft molded plastic (it doesn't scratch the walls of my shop like other extractors do, and it also wipes clean much easier than other extractors).
The list goes on depending on who you talk to and their own personal experience. In my opinion, there are probably other extractors out there that might be better, or as good, as a Mytee. For example, US Products makes what seems to be some good units (I saw them this year at the Car Care World Expo in Vegas), including one model that has a 3-stage vacuum. However, where these other manufactures fail is in price, most comparative models were 20% to 30% more expensive than Mytee.
So in the end, as long as you're getting a quality extractor (read: spending more than $500), you should be fine with most manufactures out there. Mytee simply seems to be the "best bang for your buck".