As an 'LHR guy' in this case, I have to say that the take off from LCY can be quite dramatic and fun. Is that relevant?
However, LHR is the one with the 'no changes' method of public transport. In terms of time, there's probably very little difference.
Of course, you could also use LGW!!

(But that would be longer from where you are so ignore this musing!!)