Hiya
Sounds like Stanstead is the preferred choice, however...
Luton isn't as 'hard to get to' as being made out. I'm not sure of a train link, but I'm sure there is a mainline station in Luton and from there possibly a bus/shuttle etc. Motorway M1 link makes getting to Luton easy peasy, very clearly signposted and only about 2 miles off the junction itself.
I've been to Stanstead a few times, I found that easy to get to, but quite a bit longer to get to timewise. I was coming from further in West London than you of course!!
Currently Luton is more of a pain becuase of the M1 roadworks just off the M25 and up to Luton - so you could potentially get cuaght up in traffic for ages, unless you take the side roads.
Seeing as I'm on heathrow's doorstep and that I spend so much time there with family and friends flying in/out - I prefer to pay that bit extra so it's only a 10 min drive if I ever need to go anywhere
Cheers! DtM! West London & Slough UK!