21mm should be OK, from my tests. 18mm is too wide, I do notice the difference. Also I can afford the 21mm if I want it, the larger question is would I prefer the prime, or should I just stay with the zoom that offers a bit more flexibility, which comes in handy time to time.
But better IQ and corner sharpness, plus that crazy all mechanical build would be awesome. Plus I wouldn't mind being back in primes.