Imo, the best solution is to tailor the lineup to each situation. If there's a major scorer at the opponent's 4 who must be stopped (Pau, Love, Bosh, et al), and Amare can't do it, use JJ over Amare.... if you've got the balls for that, which you should if you're gonna be head coach.
* Points allowed per 100 possessions. (An imperfect advanced stat, but it's what we have for measuring individual defense.)
( Source: http://www.nba.com/advancedstats/player.html#Carmelo-Anthony|2546;year=201112;season=r )
Jeffries is SUBSTANTIALLY better than Amare on D.
Anthony vs. Novak or Fields is not as clear... Fields has been the better defender, but Melo is a MUCH better rebounder (obviously). And we can always hope that Melo will improve his D now.... can't we?
Of course, who knows how all these dynamics will change in this reborn Knicks team? Anything can happen now. Guess we have to wait and see.