Wow, thanks for all the replies!
I'll have to have a good think about it all, but thinking JFK is the better option. The Avios (god I hate using that word!) Are not an issue as I'm accumulating them much faster than I can possibly spend them and I currently have two 241s and the GFs 30th birthday so New York is a good option for us.
Appreciate the tips on night versus day flights in regards to enjoying the first experience fully and need to think about when to fly out to maximise the experience.