Should I fly out of Heathrow or City? LON->ZRH

I'll be staying in Chelsea, London. Looks like LHR is quicker to get to, Google Maps says 25 min to LHR vs 37 min to LCY. Is the efficiency within LCY worth the slightly longer commute to the airport?

I'm going to take an early morning flight (6:45a) from London to Zurich.
The LCY flight is £18 cheaper. What do you recommend?

EDIT: Booked LCY, thank you!