Make sure the tees are 100% cotton (exclusive of decoration) to ensure the best bond with Pellon 906f.
Sulky Soft 'n Sheer Extra a little bit thicker, but it bonds better with poly/cotton blends in my experience & I think it gives a more natural look to the bonded fabric -- more like a regular cotton fabric. It bonds at a lower temp so it's safer for the vinyl decals, too -- but it is more expensive. Either way, remember not to starch/size your fabric after you pre-wash it as that affects the bonding process (ask me how I know).