Pure Convection™ uses the element that is behind the convection fan, whereas Convection Bake™ uses the element that is on the bottom of the oven and the fan.
Pure Convection is going to give you better heat distribution since it radiates from the rear of the oven cavity to the front. Heat is distributed to each rack position evenly. Pure Convection is best suited for roasting, meats, baked goods and multi-rack cooking.
Convection Bake will cook foods closest to the element first. This mode is best suited for foods that need to be browned or crisped on the bottom, like a pizza or a loaf bread.
In either case, make sure to lower the temperature by 25°F.