No, both Straight Talk and MetroPCS are MVNOs. Metro uses TMobile as their carrier. ST uses all 4 carriers - AT&T, Sprint, TMobile and Verizon. You pick the carrier you want by the color of the map on the box - Blue for AT&T, yellow for Sprint, pink for TMobile and red for Verizon.
If you "use" MetroPCS, you're on TMobile's towers, so if you want to use Straight Talk on TMobile's towers, look for a phone with a pink map on the box.