I think Ill buy MS points in order to get the GTA4: Lost and Damned DLC.
After doing some basic math, 25$ for the dlc package in store is actually more expensive than using 1600 MS Points on the marketplace to get the same dlc.
That 15% off accessories will affect both the price of that dlc pack AND MS points so you might as well get points instead.
In Canada, 20$=1400ms points & 40$=2800ms points...
So if you have a spare 200 ms points on your account you should get the 1400 card instead of the GTA4: L&D card
If you dont have any spare points you will have to get a 2800 card making your purchase bigger but at least you'll know that you got those points with 15% off...