If United is running 1-2-1 Polaris (and not 2-4-2 on the 777HD), I would stick with UA without a second thought.

DL at times runs the A330 (which has a similar 1-2-1 layout but older screens) but at the moment you are also at risk of finding either a LATAM A359 which is 2-2-2 (personally I think those planes are fine but they are a step below Polaris) or even worse a 763 which is as huge step down from the Polaris option (even the 77HD IMO)