Welcome to FT.
a) 40K SPG points are potentially worth far more than $500.
b) The $ from Drivers Edge MC can only be used for a car purchase, IIRC.
c) I hate using gift cards. Hate. I've got enough cards already. Don't need to add another 5-10.
The SPG cards are great for those who travel. Unfortunately, the options you've provided aren't travel oriented.
If you want cash-back, there are better CC than Blue Sky.