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Most hybrids' motors sit behind the transmission's gearing, so they can operate over the full range of speeds the car operates at. No hybrid that I know of has an alternator, because that's one of the benefits of a hybrid system - the motor is also a highly efficient generator, capable of producing electricity with much less loss than an alternator. All motors are also generators. Alternators are lousy generators. This is why a belt-driven hybrid would be attractive - it displaces alternator and starter both, is more efficient and reliable than either, and can improve both power and economy.
Hub motors are attractive to me in how simple the install would be, and I'm certain hub motors could be built which will operate at highway speeds. However, the most benefit of a hybrid is in stop and go traffic; if you're cruising along at a steady speed on the highway using electricity, you will run any reasonably small and cheap battery dry in short order (maybe 10 minutes to an hour for one that could fit in a carry-on bag). In the city, a hybrid motor/generator's main job is to slow your car down electrically, storing that energy back into the battery, which is then used to accelerate your car again. Rather than turning all of your momentum into brake dust every time you come up to a red light or stop sign, you instead store it up for later use.
The advantages of a hub motor over a belt-driven are that you can run on electricity alone, with the gas engine completely off, because it's an independent system, and that you could potentially get AWD. A belt driven one acts in parallel with your engine, adding power and allowing you to eliminate the starter. Both generate their own electricity, so you don't need an alternator.
EDIT: Frankly if it's a bolt-on (either belt driven or hub) you can probably hybridize your car for under 2 grand. There are basically four components: Motor/generator, controller, battery, controls (throttle). With it you'd greatly improve your city economy, reduce engine wear, and add power. However, I'm not familiar with hybrid retrofitting, even if it sounds relatively simple in principle - bolt an extra motor on your engine, and feed it power.
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