I suppose that the next lens I buy ought to be a fast 70-200, but new or used, and f/2.8 or f/4? My budget will not run to a nice shiny new Nikon with VR in either f/2.8 or f/4 specifications, nor will it run to a new Sigma or Tamron G2. I could however be tempted to buy used for around £650 to £700, but that then opens up the possibility of a new Tamron 70-210 f/4. Having just held the 2 Tamrons side-by-side at a photo event over the week-end, I can fully appreciate the lightness and ease of handling that the f/4 version offers, for basically half the price of the f/2.8. But f/4 is only a stop faster than my 70-300VR (maybe even less that a stop at 200mm), so should I rule it out? Has anyone experience of using a 70-200 of 70-210 f/4 and how this "suffers" in relation to using an f/2.8 version?