Truck sales do not vary that much month to month throughout the year. So if you just want a used one, I would focus on finding the one you want more than saving and extra $500-1000. There are only about 1 million F-150s and another million half ton GMs on the market less than 3 years old. For new, leftover 2014s will be cheaper for the next 6 months, but selection will also go down. Again, IMHO it is more important to get the truck you want than saving 2-5% of money.
I highly recommend 2012 F-150 for the 36 gal fuel tank. Also, 2014+ GMs for the 300 pounds higher payload and more power from 5.3L.