The only reason for me to consider the 120-300 is for the versatility. Otherwise I would be looking at the primes. It's a great lens, I've seen plenty of great examples from it. I haven't made a direct comparison to a prime, but without having done so, I can still say with pretty good confidence that the prime will still trump the zoom in image quality - just looking at the front element tells me that.
A good photograph is knowing where to stand.
Rule books are paper, they will not cushion a sudden meeting of stone and metal.
Ernest K. Gann-Fate is the Hunter.