Everything else equal, I would fly United in the Polaris seat - I strongly prefer the seat and that's the most important thing for me.

However, many UA lie-flat flights domestically use other, older seats that will be broadly comparable with the selection of Delta lie-flats, and UA catering is a complete disaster at the moment especially compared with DL. Broadly speaking, I would be choosing based on price and schedule first - there isn't a huge difference in experience.

In practice, I fly UA to the islands, even from MSP, on price-and-schedule concerns. DL serves only HNL from MSP, almost never has reasonably priced inventory on the nonstop, and would require an HA connection to go anywhere else, which I try to avoid. UA has tons of seats via ORD and DEN to many destinations, which usually wins out handily.

It definitely depends where you are going and what routings you are considering.