Mostly they're much of a muchness.. United's one of the very bland ones. Archetypical insomnia cure of an in-flight magazine, dull movie selections, but they wind up at the destination all the same. BA's food was a bit better than the average, but the one that's stood out has been Virgin Atlantic, with some really cool movie choices, and a 'zine worth taking away. But if you're flying with friends, that's less of a concern.

(Also, if it's of potential relevance, check what each option would cost if you decide you need to change dates. The airlines can vary quite a bit there, and even who it's booked through)