Don't buy f/2.8 zooms, buy f/4. The only reason to buy an f/2.8 zoom is if you're going to put a 2X TC on it. Then you'll have a heavy lens.
With primes, unless you have stage access, you're usually at the wrong focal length.
DLO in programs like DxO, LR and DPP bring the IQ up to the levels up primes, if you start with a decent zoom.
If you're going shoot a prime on a crop-sensor body and think of it as a "normal" lens, then at least get a 40 or a 35mm. 50mm turns into 80mm on a crop-sensor and is better suited for portrait.
The 40mm f/2.8 STM is an exceptional prime for under $200.
Buy fast cameras, not fast primes.
Dave