I regularly travel to both airports. SJC is much smaller and has the fastest TSA-pre experience I've witnessed. The 787 is also an advantage for the overnight flight.

The lounges at SFO are better (use Cathy while refurbishments are ongoing) but SJC is not unpleasant.

The only reason I consider SFO, is if I want to take the earlier flight (~16:30) on the 747, so I arrive in London in the morning. The later SFO flight is typically around 19:30 and SJC 20:10.

Getting a taxi/ Uber pickup is a significantly better experience at SJC.