If you've already decided to stay with Marriott, the best redemption value - by far - is a Hotel+Air travel package. A Marriott point can easily be worth above 1 cent that way.

There is a thread here for a membership referral - you will earn 1k points extra on your first 5 stays. (Or PM me for a referral if you want.)

If possible, use a Marriott Premier Visa to pay for stays for the bonus 5 points/$, instead of letting your company pay. (Also probably 50-70k plus 1 free night sign up bonus, no annual fee first year. Look for the right offer.)

Ask Marriott for a Platinum Challenge. (There are several threads here about it.) They will often offer you Platinum after 15 nights. Then you can earn the Platinum bonus points and amenity points.

The airline choice is a separate issue. Probably best to choose one airline that you enjoy flying and which also has the redemption opportunities/costs that meet your travel goals - then stick with that one airline as much as possible. Even if you don't care about upgrades, any elite status is far better for flying than none.