Depends - for us we used LHR this past July because we could get economy comfort of the flights and still use 30,000 less points per person than flying to LGW regular seats. Additionally we were staying at Hyde Park area so there was no concern about rushing to port. When we returned to port, while LGW is presumably 10 minutes closer, several
DCL busses heading for LHR left prior to the first bus leaving for LGW. When we hit traffic I don't think it really mattered where you were going, there were a line of trucks nearly 10 miles long headed to Dover, reminiscent of the truck lines from Canada to the US after 9-11. They were on the other side so we just had normal morning rush hour traffic.
We will be using LHR again this time, mostly because we can get direct flights and they are less points than LGW.