UA will be friendlier with flight credits should you cancel. You'll have one year to use. Swiss won't be as lenient.

To determine PQP, use:
https://pqp.economiles.com/
Odds are you'll get more PQP booking on UA.

I think EU flight cancellation rules will be by operating carrier so the booking carrier doesn't matter. Swiss metal to/from will be covered. United metal from Europe (but not to) will be covered.