Mechanical no question. Robotic derailleurs are overkill.

Will the Di2 parts all still work in 5/10/15/20 years time ? Most of the mechanical bits will, only the internal STI shifters are likely to develop in issue. The brakes and deraillers if they are not destroyed in a crash will last a looong time. And if you need cheap spares in a hurry, 105 and ultegra bits are compatible.

Who knows if Di2 spare parts will be available in 5/10/15/20 years ? My prediction is Shimano will eventualy, in the coming years, adopt wireless and leave the wired Di2 groups in the dust along with the customers. Just like they did with the first version of Di2 7970.