Spent a happy 10 months doing LGW - IAH once a fortnight, interspersed with LHR - Lagos. Am now doing LHR - CAI once a fortnight.
LGW long haul in my opinion has a much more consistent and on average better quality of service than flights out of LHR.
LGW is horrible to get to / from, but the check in / security is better than T4. Agree the lounges are rubbish. Punctuality is way better than LHR.
LHR is horrible, but the lounges are better than LGW.