It all comes down to what you intend to do with your car, and how much you have to spend.
If you just want the cheapest replacement turbo, go with a plain Jane 16G. For those of you with 89-94 cars, it bolts right up and it's under $700. Even for 95-99 folks who will need additional hardware, it's still a good deal compared to a factory turbo, and it will really improve the top-end power.
If you want to autocross, you'll want as much low-end grunt as possible. Stick with a factory turbo or small 16G.
If you're a drag racer, you want high-end power. Go with a larger turbo, and damn the turbo lag. If you can afford the whole setup and this car is not for street use, the Mitsubishi 20G, combined with an external wastegate and proper fuel delivery, is a proven power-producer.
If you are road racing, you can also stand some lag, but you may want some flexibility so you don't have to shift all the time to keep the turbo on the boil. The large 16G will probably fit the bill.
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