flyingtabby,
"...and start at a UK airline with the eventual goal to move to the US together."
I'm afraid I don't have an answer to your question about which path is better for you (I have an unqualified one but I'll keep it to myself. Someone more knowledgeable than I will eventually comment.)
Just idle curiosity on my part but what's your plan for getting the legal ability to live/work here once you're competitive for a US airline ? That's the underpinning of your entire plan.
As it stands now, you'll need a Green Card and as far as I know, there are only 2 ways to get that: win one in the Green Card lottery...or marry your girlfriend. I'm unable to comment on the availability of other forms of visa or whether any US airline will be accepting such visas in the future.
Best of LUCK (not muck
) to you in any event.