i would say that out of them all the v6 is obviously the most complete nxt tactic at the moment, good in any situation but ..
if you want to see many goals, like 7-0 to city vs barca style results on the other thread. you need to use all of the tactics when you need to based on the opponent game style and str.
v2 v3 gives you way way more wing play, a lot more crosses, against barca style of teams, teams that play via the middle massively.. this should do great.
unlike v2, v3 suits more for the low teams that don't have paced defenders.
v5 offers more control, more deadliness overall and a bit improved defense vs the lower versions. but it cuts on the cross ends and the wing play for a more narrow control type of game with fast passes, and very good football to watch overall.
now with the v6 i went a step back, i first had to deal with the here and there goals that have been consumed so i lowered the tempo of the game, i lowered the close down and the retrain pos to make an overall much more stable tactic both in attack and defense. the football may wont look as good as the v5 thou it still will look great anyway. but it offers much better defense, more goals per shot, more ccc's , more goals from crosses and overall better control on the field.
your choice what to use, i use them all when needed, finished a perfect season