Mileage benefits -- all that matters is the operator, who markets the flight is not a factor.
Operational issues -- in this case the advantage for booking tends to go with the flight operator, avoiding codeshare.
If mixed operator, go with the one with the most segments or the one you you have most concern oprational issues.

one exception if you have a UA EQM/PQM earning credit card -- then consider purchasing on united.com.