The best flights are internationally configured (3-class) planes. Upgrading to international business is So much nicer than regular domestic 1st class.

These flights also have lots of seats in the C cabin, so *should* be easier to upgrade. A 2P might even have a chance.

However, as a 2P, I would try and upgrade any flight where I thought I might enjoy the 1st class experience, because you won't always clear, and you don't get anything when your 500s expire.