12-speed mechanical 105 shifts too nice to upgrade
personal opinion. it shifts even better than 11 speed Ultegra. I still debated for quite a while if it's worth to spend extra money for changing to di2 Ultegra or 105. I tested both electronic groupsets. The biggest improvents imo are the servowave Ultegra brakes and the more premium feel of the lever.
in terms of shifting performance I don't think there is that much difference (assumed that everything is properly setup)
shifting into higher gears works fast with the mechanical lever as well, however shifting up the cassette into the lower gears is simply better on the di2 setup (because of the long way you need to push the mechanical lever inwards). - that is the biggest advantage imo.
when it comes to the front derailleur the mechanical 105 shifts great. It still has that half click (trim) mechanism in case of chain rubbing, but overall the chainring swap works fast and smooth.
just my 2 cents. because many people who own road bikes have been using electronic shifting for years and wouldn't bother about a mechanical 105 (understandably).
on a budget however, I would 100% use the money for a set of carbon wheels, instead of choosing di2 and sticking with heavy alloy wheels.