I would rather have the extra light from the 50mm, and then crop later if needed. You can always crop in from the 50 but there's no way to get more light out of the 85.
How much MORE light will you get out of a f1.4 vs. f1.8?
How much more MONEY will that light cost?
How many of those shots will have blown focus due to small DOF?
This is a common misunderstanding about the rationale of large aperture lenses.
The main reason many pro photographers buy low-light primes is
not to use it with the largest aperture possible (i e 1.4 or 1.8 ).
No, the real benefit is the fact that they often are much
sharper than their entry-level counterparts
after stopping down 1, 2 or 3 steps!
So if you're able to take concert shots at 2.8, those shots will be noticeably sharper using a Nikkor 85/1.8 (stopped down to 2.8 ) than a Nikkor 85/2.8 at maximum aperture.
But you have to read up on your particular lens' characteristics. Some low-light primes can't compete, even though their maximum aperture is larger. Bring a Nikkor Noctilux (50/1.2 or even 1.0) to a show, and it might be terribly "fuzzy" around the edges even after stopping down, compared to a 50/1.4.
Personally, my all time favorite concert lens is my Nikkor AI-S 135/2.0 - alas no AF though.
Using my Jedi skills, I can get sharp handhold shots at 1/30 sec, which allows me to stop down to 2.8 or even 4.0 depending on the light show and my ISO.
So those of you planning on grabbing a 50/1.4 or 85/1.8 only for the purpose of squeezing out the most light out of it and shooting at 1.4 or 1.8: rethink your usage pattern and dare stopping it down, to get value for your money spent.