Just to try to be clear here.

Delta Comfort Plus is an Economy Class product. It features Economy Class seats with slightly more legroom and slightly better drinks and snacks. The equivalent United product is marketed as Economy Plus, and generally includes somewhat less than Delta Comfort Plus (same food and drink as normal Economy, no priority boarding, but same extra legroom).

United Premium Plus is a Premium Economy Class product, generally only offered internationally. It features different, wider seats with more pitch (roughly similar to domestic First seats) as well as plated meal service. It is a significantly better product than anything in Economy, including Delta's Comfort Plus. The Delta Premium Economy product is marketed as "Premium Select". In general, I think people find it inferior to United with slightly less well presented food and most importantly an extremely inferior 2-4-2 configuration on the A350 which is a very narrow PE product.

There will be a night and day difference between the two classes of service. Difference between the carriers can vary and may be a matter of opinion.