I have both and I ride both frequently.
I'd say I probably spend an equal amount of time on mechanical and Di2. I like both and they both shift extremely well, but Di2 is oh so sweet and such a luxury to use.
Put it this way, if Shimano Di2 were completely wireless, I'd upgrade my (externally cabled) mechanical shifting bike tomorrow.
If it's in your budget, go for the electronic groupset. 😎