Having acquired two more powders from MAC, I tried to figure out the difference between these two, along with the advantages and disadvantages of both.
Select Sheer Powder makes your skin matte and creates a soft focus finish for your makeup. The name may be misleading, because this powder actually has some color to it (mine is NC25). My makeup artist told me that because it has color, it can be useful for when you need to set your makeup. It can also be great for issues with wrinkles, because it has a special coating that creates a look that minimizes the appearance of fine lines. If wrinkles are a concern for you, I would probably go with this product.
Then my makeup artist explained the Blot Powder to me. She told me that it, too, is great for setting makeup, but without adding any color or texture. And it is especially advantageous for people with oily skin, because it is geared toward shine control. From my own experience with this product, I agree that it is great for shine control, but I will have to disagree with the claim that it does not add color.
I would argue that there IS color in Blot Powder, and there is also a color difference between when you first apply it and at the end of the day: it is lighter when you first apply it, and by the end of the day it gets darker from absorbing all the oil on your face. Currently, I have the Medium Dark version of Blot Powder, but I am thinking about switching it to Medium because of its tendency to get darker over time. I am kind of disappointed in this product because of this issue.
I guess it all depends on what your concern is. If you are worried about wrinkles, I would go with the Select Sheer Powder. If you have issues with shine control, I would go with the Blot Powder (but I would go with one shade lighter because it turns darker).
Personally, I do not use either of these products right now. They are sitting in the back of my drawer gathering dust because I am setting my makeup with Studio Fix Powder Plus Foundation, and if I need any touch-ups throughout the day, I bring along my Studio Fix Powder Plus Foundation. Eventually, I may carry around the Blot Powder with me instead, but right now I have it in the wrong color. If you are interested in either of these products, they are shown below: Select Sheer Powder is on the left and Blot Powder is on the right. You can get the Select Sheer Powder and the Blot Powder for $21.00 each at www.MacCosmetics.com.
I hope this helps you with your decision between the two products. Please let me know if you have any questions or comments. And thanks for visiting!