I should also mention if you want an inexpensive option with reach that is easy to sneak in alongside the pink kr try an old M series 200 f4.0 it's easily fit in a pocket and does well as a concert lens (it's the 200 i use at shows actually) It takes a bit more to manually focus but It's not hard to get good shots with something like that
this is in a really poorly lit 100 clubin london in a big crowd shot held up high using the highest iso my K10 was capable of 1/25 sec at f 4.0
not the best shot but it shows even in crap light f4.0 is doable even with a sensor that realistically shouldn't be shot over iso 640 or so