I have owned both and decided to keep the 300 f2.8, for what I photograph. However, it really depends on what you want to shoot most of the time. If you're doing field sports a zoom lens is very nice to have. The 200-400mm works very well at distances less than 100m, so if that meets your needs it's the way to go. Both are close to the same price used for a VR I version, the 300 might be a tad less now. If you're usually shooting at the long end and need the reach but can't afford a 400 f2.8, the 300 f2.8 with TC's is a better choice. The 200-400 doesn't do as well with TC's and you can get better results with the 300 f2.8 / 1.7 TC than you can get from a 200-400 f4 / 1.4 TC.