Another way that will be benificial when (and if) the Diner's Club/AAdvantage promotion starts:
MR to DL Skymiles: 1 to 1
DL Skymiles to DC: 1 to 1 Club Reward
DC to AAdvantage: 1 Club Reward to .625
AAdvantage Miles
All boiled down, your original MR point would be worth .625 AAdvantage miles, while your original MR point through SPG would only be worth .333
tovaz