you forget about one thing, you can get war caster only from feat but you can get con res from items that why war caster is better
You can get con saving throw advantage from items as well. Steelwatcher Helmet, Barkskin Armor, Dark Justicar half-plate, to name just 3. And con saving throw advantage is better than advantage on concentration saving throws, as it applies to concentration saving throws and more.
And yup, there's an opportunity cost to using items, but that also applies to feats.
Anyway, War Caster is generally better for keeping concentration, as well as a choice of how to react to enemies moving away from you. Resilient (con) though helps all con saving throws, so helps against things like poisons, and also gives you a point of con.
