SPG: Convert points into miles: SPG members may convert SPG points into miles at a ratio of two points to one mile.
Not Worth it.
Marriott:
Convert points into miles: With RewardsPlus, all MileagePlus members can convert Marriott Rewards points into award miles at a 20% discount compared to other airline partners. The following conversion rates apply:
8,000 points to 2,000 miles
16,000 points to 5,000 miles
24,000 points to 10,000 miles
56,000 points to 25,000 miles
112,000 points to 50,000 miles