okay guys, LOW LIGHT concert photography&dark occasions, etc. sigma 30mm f/1.4 or 70-200mm f/2.8? could use the zoom&speed since my 200 now is f/4. or is it a better investment to go with a prime? i get VERY close to the stage, but i havent seen if a 50 could do this. already have the fifty. any suggestions? pictures? thanks.