I'm making a switch from Nikon APS-C to Sony full frame and am about to buy the Sony A7RivA. I have the budget for the Sony 24-70mm 2.8 GM, but have seen a number of reviews that say the Sigma is sharper than the GM; which is a major consideration when the 61mp will highlight every flaw in a lens. My main concern is image quality and autofocus. I'm not too bothered about how heavy the lens is. If you had the choice, budget was not an issue, but the ability of the lens to match and maximise the 61mp of the camera; which of the two lenses would you pick?