Would it be worth it to sell my Canon EF 28-135mm IS USM w/ hood and 72mm Hoya Super HMC UV filter and buy a Tamron AF 28-75mm f/2.8 XR Di or Sigma 28-70mm f/2.8 EX DF? Money is not an issue (I'd make like $50-70) and the extra range isn't that much of a concern since I have a Sigma 70-300mm f/4-5.6 APO Zoom Macro Super II to regain the 71-135mm range (and I plan to move to a 70-200 f/2.8L when I have the scratch sometime down the road). So basically it comes down to: would there be enough of an image quality increase and is the constant f/2.8 worth the hassle of selling my current lens? TIA.