I think it makes more sense to think about aircraft types than airlines.

For example, if I'm not worried about United miles at all, I would pick a A330 or A350 over a UA (or any) 10-abreast 777.

And if you aren't chasing UA RDMs or PQMs, there's no reason to limit your self to Star Alliance. In any case, discount economy on many Star Alliance partners earn little or no miles on United.