All good points, but if you are also trying to obtain points to reach one or more of the Marriott lifetime status levels, the 5x points earned using the Marriott card will count toward lifetime status but the 6 points earned using the SPG card and then transferring the points to Marriott will not count. In addition, if you use the Marriott card, the charges will help you obtain credit for a Marriott stay while the SPG card won't.
If I was already platinum due to my prior history or due to my ownership level and didn't have keep track of stays and lifetime points, I'd use the SPG card.