I vote for strips. You'll get more usage out of them and even though 20 shirts seems like a lot, it probably isn't enough to make more than one bed sized quilt--2 good size throws unless you add more fabric. I would cut them into 2" strips and just make log cabin, rail fence, 9 patch type blocks with all of them and then decide how many blocks to use with filler fabric it will take to make however many quilts you have planned.