8v is one injector, 80 hp.
16v is 4 injectors, 95 hp.
So 16v will be a bit faster - as well as having 3 more injectors and more valves to worry about. Personally, I have the 8v, and, with it tuned and timed properly, find it peppy enough to get me through just about any situation.
The transmissions come in 3 speed auto, 4 speed auto and 5 speed manual. Again, my preference is the 5 speed manual - a bit better gas mileage, and I personally prefer driving a manual. The 3 speed auto is totally mechanical, and an extremely dependable tranny, having been used on postal trucks, etc., for decades. The 4 speed manual is more complex, is electronic, is shifted by a tranny computer, etc. Basically, the 4th speed is an overdrive.
These cars, particularly in good condition, are getting harder to find. Rust is the basic issue, and if you find one for sale, make sure that you get under it and check the frame, particularly by the rear suspension and gas tank where they are prone to rust. Frankly, it is somewhat unlikely to readily find a Trackick with the tranny, top, year, engine, etc., that you want, unless you are extremely patient. I imagine that you will have to compromise on some of the things that you would want to have.