How would a Sony compare for sports use to replace my Sigma 70-300mm (the one with the now stripped gears, grrr!). I got some cracking rugby action shots with the Sigma, but it's now bust and I'm loathe to go back to one. Considering an 18-250 / 270mm Tamron or Sony if it can be had for a little bit more money (depending where you look) than buying the Tamron or Sony 70-300mm. WOuld also allow me to flog the Sony 18-70mm kit lens...