Two very different races. Here's every stat, side by side, to help you pick.
Indicative — based on GPX data. Both courses normalised to start at 0m.
✓ marks the better number on that stat.
London is a World Marathon Major with a current world record (1:59:30) and a field pushing 60,000; Valencia is a non-Major that has quietly become one of the fastest courses on earth, with its own strong 2:01:48 men's record and a course built with almost no purpose but speed.
London carries more elevation change (85m of gain) and a bigger, louder occasion along the Thames; Valencia is flatter (62m of gain), smaller (around 30,500 finishers), and finishes on a blue carpet laid over water by the City of Arts and Sciences. If the Majors prestige and scale of London appeal, that's the clear pick; if you want the flattest, most single-minded PB course of the two, Valencia edges it.