Additional pro: if you use Economy (which I'm doubting in your line of work) if you choose UA MileagePlus is Economy Plus for free once you attain Silver status and can reserve farther ahead with Gold status. It's still regular economy (unlike SAS Premium) however there's a significant amount of extra legroom.

I think biggest pro to North American programs is non-expiring miles as long as your account is active. Personally in lean flying years I redeem very few miles, so nice to be able to keep them for an extended time.

Con to UA MileagePlus, need 4 segments a year on UA metal to maintain status, assume you'll use UA while in the US but after you move again perhaps not. AC program has the same problem, but requires even more segments flown on AC metal. I do not believe US has this problem, but then no UA E+ access with US status.

For redemption, US is more favorable on a purely miles spent per ticket basis and I've found availability to be basically the same, however no one way awards, UA and AC both have one way awards and US is pretty inflexible on changes.

As for credit cards, the US credit system is a little tough to crack into quickly and affinity cards (eg. mileage cards) usually require more established credit than some other cards. With only 3 years, I wouldn't count on too many miles that way... but Amex is a great option for you since you can automatically have your card from Norway transferred to the US and the US Amex Membership Rewards does give quite a few options for mile transfer.