I have both. the 70-200VR is a great lens. In very aspect: huge price, huge lens, huge weight, but most of all: great image quality. I bought the 70-300Vr as a somewhat lighter walkaround tele. It's smaller and lighter and it's rather cheap. The 70-300 amazed me. it's very sharp even wide open. Sure my 70-200 is sharp too, but @ 2.8 it's a bit soft. So i always stop down my 70-200 to get that sharp look. The 70-300 is sharp enough wide open. So it boils down that my 70-200 is collecting dust at the moment. 2.8 is nice, but it's 'only' a 1 stop advantage. i simply bump the iso by 1 stop to get the same shutterspeeds with F4 compared to F2.8. I'm just not sure if you will get enough bang for your buck if you get a 70 or 80-200 2.8. It's up to you, the bokeh on the 70-200 is superb tho...
and to be honest: if i have to get it really right, i grab the 70-200. Somehow it has a slight edge on any picture you shoot with a lens like that...
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Raymond Francois