As with all things, you need to balance cost against comfort against convenience.

For KLM and UA in economy, I would choose KLM. If you are entitled to or willing to pay for UA's Economy Plus (just extra legroom), I would choose that over KLM.

If you are willing to pay for Premium Economy, then it's UA - and UA's product is one of the better ones out there.

If you are willing to take a connection, you need to consider how much you need to save to make it worthwhile to add the extra time and hassle involved. Also, I would choose airports where connections are easy rather than difficult. Avoid LHR, CDG and FRA like the plague. ZRH, MUC, CPH, LIS and DUB (I think that's the list from SFO) are ok. With current pricing, I would be surprised if you saved enough to make the connection worthwhile. And none of those airlines have seats or service that are markedly better than UA's or KLM's.