That is weird.
You don’t want to bring your S2 to gigs because you don’t want people to steal/damage it. Ok, that’s understandable.
So you look for a cheaper controller.
However, no matter what is the controller, you’ll bring a MacBook Pro, which is way more expensive than a S2 that is worth roughly $400.
Why don’t you simply use the club/venue setup then ? 100% safe, nothing is yours except few CDs or USB sticks.
true… the controller is the cheapest part of the setup. well an S2 compared to a mbp anyway. if u could find a slightly battered+cheap S2 then that would be the best option i guess, if u really don’t want anything to happen to your main S2. i’d find having different controllers more appealing than two of the same type tho - unless they were one deck controllers ofc 
I bought air in december and it has some flaws, but I basicly like it. Mixtrack Pro is very popular even among pro users so I’d still go with that.
When using Dj Control Air, you won’t accidentally use proximity sensor. Then again, it’s a bit hard to change effects on air because there’s a little lag and using proximity sensor is a bit tricky in Traktor as default. If you want to use effects and you use them with knobs, adjusting them from 0 to 100% accurately, you won’t be able to do this as you like on Dj Control Air. It’s like using volume knobs with wireless mouse, sometimes you manage to succeed, sometimes mouse loses signal for a second and master knob goes +134152452. Then, if you just want to adjust effects before using and then push a button, it works as it should.
My personal opinion is that Traktor autogain isn’t working well enough, at least if you play new and old music mixed, There are no gain knobs so I remapped +/- -buttons for gain adjusting. Now it works decently, but I still would prefer knobs.
The best advice I’ve heard, is to get a nice flight case with a laptop deck for your Kontrol S2. You are use to practicing on it, so it will give you the best live experience. Plus, the S2 (just like other controllers) will be outdated in a few years. It won’t be “your precious” for long. That’s the digital truth.