If you're looking to add to your flash set up Yongnuo have announced a new flash that has built in radio AND full integration with Canon's RT system. Might make a good cheap alternative.
Also, you seem to have a bit of an unbalanced gear set up with an expensive L and then bottom of the line rebel. What kind of concert are you shooting? Is this a paid thing? I'd spend money on getting a better body first. Yeah I've heard all the bla bla bla about lenses being more important. I'm sorry but there IS a difference between a consumer level crop camera and a pro grade full frame body. If you're serious about photography (and you seem to be) why give yourself such a handicap?
Back to flash - you can do LOT with just one light.
Get the camera sorted out first then add to your lighting.