I have just recently started shooting with a prime lens. I used and still do, a 100-400 zoom I have gotten excellent results with the zoom. You have much more flexibility with it. There are a couple of big differences. The prime lens has made me a better photographer sounds a little far fetched but it's true. The prime lens forces me to set the shot up better it slows me down getting it set up but that time allows me the time to take more things into consideration lighting, angle, background ect. As far as being sharp at f8 and above my 100-400 is very sharp, I would recommend that you get a zoom with LD glass it will cost you more but in the long run you will be very happy with the difference. I'm simply amazed in the difference in color between lens my prime lens (LD) color qualities are vastly superior to the zooms I mean night and day. Bottom line if you working on a budget get the zoom maybe a little longer than 300mm but set a side a little savings account for a prime lens down the road then you will have the best of both worlds. Randy