If you're absolutely certain you will buy an a7-series camera, you might as well get a compatible lens now. The Tamron 28-75mm f/2.8 G2 is an excellent option, as is the Sigma 24-70mm f/2.8.
But if you still haven't made up your mind entirely, then perhaps it's better to get a lens that will be more useful for you now. Buy one used, so that reselling it later isn't as painful. Consider the Tamron 17-70mm f/2.8 and Sigma 18-50mm f/2.8.