If your current groupset is near end of life it may be worth considering an upgrade to 6800, or 6870 if you want Di2. Otherwise, you're replacing good components with only slightly better ones. If you're hellbent on picking one over the other, I'd go with the power meter or possibly Stages power. Depending on how you ride and what your goals are, a power meter will help you focus on training. You can also do this with a heart rate monitor, but a power meter gives more accurate information.