The problem with the bus it that it uses the M25 motorway, one accident and you are stuck.
It happened to me last year. Luckily I was still at LGW
waiting for the bus that never showed up.
So I took the Gatwick Express to Victoria, outside an onto a bus to Paddington, and then the Heathrow Express to LHR
. Very fast and only took an hour total. But heavy on the wallet (luckily my company paid), but worth it as I saved a nightstop in London.