In a medium large bowl combine shortening, margarine, sugar, brown sugar and cream together until well combined.
Add the eggs, and vanilla and mix until fully combined.
Add the flour, baking soda, and salt and mix until the dough is formed and a good thickness.
Mix in the chocolate chips and M&Ms.
Roll into 1 inch balls
Cook for 7-8 minutes or until the bottom is golden.
Notes
If you had to replace one or the other, I would recommend you replace the margarine with butter. The shortening is what is really going to make these cookies soft, and chewy, and give them the better taste.