I can only comment on Star Alliance, UA is obviously the *A choice LHR-ORD. I (and I think many others) really like UA's flat-bed business class seats.

They just introduced the re-branded Polaris business class. New seats are going in slowly, so you'll likely still have the current seats. But the food and service should be the new Polaris offer, which I haven't tried yet as it was just rolled out in December. You can read more in the United forum or on the United website.

The old seats mean there won't be 100% aisle access, that's one of the negatives for UA if that matters to you.