In my opinion in the yr 2012 the difference is a TAG will attack regardless of hand quality in very good spots/// meaning a TAG can squeeze in a great position or 3 bet uber light, but does so aggressively in a spot that isnt marginal.....while a nit is just too tight to take advantage of situational poker very much.
Nits did well back in the golden age of online poker(2002-2006), but since the proliferation of HUDs, coaching sites, and general poker knowledge, they have proably become the most exploitable player type out there.
Honestly, nothing is more fun than playing my semiLAG/TAG style against a table full of nits who think they are going to trap me in one big hand.
Obviously I have their stats and a strong read, so unless a cooler situation comes up it just isnt happening.