Do not go just for the bonus miles.
If you have UA miles but your flight is blocked, you can't use them. Much like DL.
If you have LH miles, you will need a lot relatively and can't really buy much to get an award.
US you can buy a lot easily, but they can freeze your account!
Asiana allows you 2 years to get 40k miles to get Diamond status and that allows for lounge, extra luggage etc for 2 more years. With an Am FFP, you have to requalify each year.