I flew UA CLE-EWR-TLV last year, CLE-EWR in F and EWR-TLV in J. Here is how I would compare it to DL:

• UA F was dark and dingy. The seats felt very old, and had no seat back screens. They had no snacks, no PDB, and only one pass for drinks in the air.
• Polaris lounge was fine. The restaurant was nice, but slow. The rest of the lounge felt like the best examples of Sky Clubs, but not really any better.
• Polaris J hard product seemed like a fairly standard reverse herringbone J seat. Not significantly different than DL’s.
• Polaris J amenities are better. Bedding, pajamas, and amenity kit all beat Delta.
• Polaris J food is terrible. Pretty much everything is brown lumps served in brown sauce.
• Service in domestic F is much better on Delta, service in J is a draw between the two.