The rev/mon kit will probably be the easiest to find. Only avoid it if you're allergic to raised panel lines. Replacement cockpits are available - and, of course, if you plan to do an aggressor aircraft, fill in the slots for the wing pylons. Don't know of any Aggressor F-5s carrying gun pods or LGBs. They're also (IIRC) the only game in town for an -F model, currently out of production.
Haven't built the AMT/Ertl. Actually couldn't find any reviews on it, either, except for one "I built one and painted it as an F-5A, which is wrong" review. (Honestly, that's all they said.) Going through stuff on the F5fans group at Yahoo, it mentions "numerous fit problems" and "Detail lacking."
The Testors kit is of an F-5A. Can't think of any -E or -F they've released, and there are definate differences.
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for one review.