It really depends on your objective. If it is in-flight comfort then UA's E+ (and exit row access) is probably your best choice. None of the US-based carriers offer a very good soft product, i.e. service, food, IFE, in the economy section. Op-ups are a crapshoot everywhere.

However, if you actually care about using miles for international business class tix or upgrades I would NOT recommend UA which blocks flights and is generally stingy with advance inventory (see *A blocking thread in UA forum).

Instead, you might consider BD which provides all of the same *G benefits listed above (plus access to UA RCCs in the USA) and offers much better TATL redemption.

As you are based in BOS you might find AA to be a better choice. As AA Gold I have found it easy to book exit rows on AA so you would have no probs as Plat, and would get roughly the same benefits as UA mid-tier including lounge access. Plus, AA does not block partner award inventory as UA does.