I have a DD at both LEH and LU, so a friend suggested I post here. I agree with most of the comments made above about the schools so will try not to repeat too much..
Both schools attain similar academic results year on year, and I would not read too much into any isolated year differences. If LEH had a 'strong' GCSE year group this year, LUs was reputedly less so. A couple of subjects did not perform as strongly at LU last year but I believe this is being addressed. IMO, LU has a more widespread academic intake on entry. Teaching at both schools ranges from inspirational to more mundane, but LUs maybe reflects the CoEd element and seems slightly less prescriptive overall. They do however provide slightly different GCSEs and A Levels/sixth form programmes, so check there is nothing eg language wise, you particularly like/require. The new LU 6th form programme looks terrific.
In terms of homework, LEH sets more subjects per night with a lot of revision for their frequent tests. LUs seems well considered on most occasions and there is less of it. Homework needs to be completed on time and there are penalties for not doing so.
On maths sets - LEH sets slightly earlier than LU, but by GCSE they are taught in sets/bands. In LU, the top band (maybe three classes) sits the GCSE early, and the middle (3 classes) and lower (1 or 2) in the summer. Both schools move pupils between sets to get the right fit for them. Most students in both schools will get A*s or As.
In science - LUs expects most students (girls and boys equally) to sit triple science at GCSE, whereas at LEH they are given more of a choice to sit the double award.
There are plenty of clubs at both schools, and pupils are encouraged to try new things out at both schools. It's a good opportunity to meet like minded people and extend friendships.
Phones - both schools have strict usage policies which the kids do their best to get around. They are usually one step ahead of the school.
Transport - we are half way between the two, and transport is easy for both. LEH has a late bus covering many areas, but the A316 is busy at many times. Many clubs/matches/rehearsals for music and plays etc are after school so even with lift shares you will probably need to negotiate the traffic at some stage. Dds in LEH are put in classes with geographically close children where possible so if you are lucky, their best friends are nearby. The tube/bus has been very reliable for LU and DD became independent very quickly at getting herself around west London to friends and venues.
Self motivation is important to get the best out of both schools, but I feel LEH was more nurturing in the earlier years. My LUs DD initially thought the boys were 'annoying and disruptive' (her words - and some of the girls were as bad), but now older, values them as friends and has a fabulously mixed social group.
LU is more cosmopolitan, reflecting it's more urban setting, location and intake. DDs friendship group is like a mini United Nations. There is diversity at LEH but less so. LEH DD loves the open spaces, well manicured grounds and relatively fresh air.
Sports - teams/clubs for all abilities in the early years, but easier for an enthusiastic sporty person to get into the top teams at LU due to less competition. LEH provides more traditionally 'girl sport' - gym, trampolining etc, more timetabled sport, and in my view, better coaching in most areas. LU has however recruited specialist coaches eg netball, addressing some of the issues. Girls at LU often play sport onsite or at Chiswick. Both schools have fabulous new sports facilities and no major building work/ disruption planned in the future for either school.
Both schools have great theatres and are strong in music/drama/the arts. IMO, LEH provides more opportunities to get involved and at all levels.
Both DDs love their schools and would not swap, whilst acknowledging the strengths of each other's school. Transition for LEH DD was easier, (LU were very helpful in trying to resolve the issues once raised), but this probably reflected their personalities. One chose by listing the positives and negatives for each school (not many negatives), and as the academic results are so similar, just went with the one she felt most at home. It's a nice dilemma to have, and at the end of the day, if your DD feels she has made a mistake it's usually possible to transfer between the two schools.