Have opportunity to book non-stop business class from Phoenix to London-Heathrow. The only two carriers (non-stop) are British Airways and American. Prices are about the same.

Which might be the better experience considering seats, lounges, etc... ?

Hi: I can't yet speak to Business class on AAvsBA from PHX. However, I can speak to BA's reliability on the LHR - PHX route. It's problematic to say the least. I've had three flights either cancelled or delayed by a day - two due to mechanical issues; one to a strike. I had a fourth flight where they switched out planes and I was bumped to business from first class because the new plane didn't have first class. I'll be flying AA this May but, P.E. not Business, however I have been pleased with the AA business product ( most recently LHR - DFW with connecting AA flight to PHX from DFW). Concerning lounges, there are three AA lounges in Terminal 4 versus the one BA lounge. Also there is a new AMEX Centurion and an Escape lounge in Concourse B, Terminal 4. Hope this helps.

One advantage of travelling on a flight operated by an EU or UK carrier to Europe is eligibility for compensation and right to care if delayed as stated in the regulation.

Personally, I prefer AA, as then there is no need to jump over anyone or be jumped over. But cabin crew service is better on BA.