Every photographer regrets buying the cheaper lens. You will always end up selling it to get the better lens, at a loss.
I always wait until I can afford the "L" lens.
The Canon 70-200 L IS is my "go-to" lens for concerts! The only thing better would be the f1.8 300mm, which is twice the price and also leaves you stuck at shotting from rather far away -- 300mm up close would give you nose hair shots.
Get the fastest lens you can afford... the f1.8 85mm is a great lens... but it is a prime lens "stuck" at one length... hard to vary framing with a prime lens... I like the range of the 70-200 better. The f1.8 85mm is also more money! That lens is the best lens for close portrait shots.
Also, if you know you are going to be close to the stage.. a "cheap" f1.8 50mm prime is a great lens. Super fast lens... plenty of frame area up close... sharp... lens goes for under $200! The f1.4 is even better... for under $400.
speed... speed... speed... low light... low light..... artists are moving about on stage. I see so many people where all their shots are blurry because of too slow shutter speed. my 2 cents.
This shot is from up in the balcony at the Beacon:
spec: 1/125 sec 800 ISO f4 at 200mm with the Canon 70-200 f.8 IS. Anything I shot under 1/125 would be slightly blurry. And that's with IS... so without the IS you would need to shoot at least at 1/250.
So any lens slower than a f2.8 and you are right on the edge on not get ANY good shots... a f4 lens is too slow unless there is "a lot" of light on stage from the balcony... don't forget your depth of field even at f4 on a f2.8 lens is only about 3-5 feet!