The pictures of the 17-55mm vs the 50mm speak for them self. The 50mm looks much better at F/2.8 than the 17-55mm F/2.8 at the same aperture.

How are the pictures prepared?

• tripod
• same exposure settings
• two flash setup
• taking pictures at F/2.8, F/4,F/8

The prime is at this point already stopped down, but to make clear. I do not want to proof that a prime is better than a zoom. This is obvious.
I want to make clear for my self, if I can justify to keep the zoom or if the prime is good enough.

Well the pictures make clear, I'm more happy with the 50 and can live with the inconvenience of swapping lenses for my goal. It's also so much lighter to carry several primes with you than this zoom.

conclusion

• the prime is much sharper
  
• the zoom has a nicer out of focus rendering, but the prime is pretty close, but not as smooth
  
• the prime has much better CA values (at F/8) and the CA is the same at F/1.8
  
• the zoom has better contrast

There is no reason to use the zoom at 50mm instead of the prime. So the prime wins.