OK, true but for City that's a really small part of London: the Totenham Hale train station and its immediate vicinity. It's 46 minutes from Totenham Hale to LCY and 34 to STN. Anything further into London will win for LCY.

We recently flew to Gatwick and I was pleasantly surpised how quick everything was. Barely any waiting times, the hardest part was figuring out which train to London needs what ticket.