Well.. You’ve named 2 high-end, pro spec controllers and a slightly less pro spec controller in that bunch.. (the SR being a rung down)..
All of these have stand-a-lone functionality, decent audio interfaces etc.
Advantages of them (from my eyes):
NS7:
+ Spinning Turntables – apparently easier to scratch on if coming from vinyl but a bit of a gimmick otherwise
+ Good layout, (Personally I love the idea of touch sensitive EQ knobs for easy killing..)..
– Unbelievably huge and heavy… not portable..
SX2:
+ Good layout, good feature set etc.
– Quite large (though not as big as the NS7)
SR:
+ Smaller controller
– Only 2 channels
Basic summary.. all are decent controllers but all have their own features and plus points – decide on what you want most for your style of DJing.
bob6397