I had the 300 F2.8, and replaced it with the 120-300. Nothing wrong with it, IQ about the same to my eyes, but the added flexibility of the 120-300 swayed it for me. I like to be flexible when I shoot, hence no Primes whatsoever (I know - sacriledge)
Also, if Size and weight matter to you, the zoom is a whole lot bigger than the prime.