cotton or poly what???
batting? fabric? thread? clothes?
each has it's place-
i use both cotton & poly batts & blends & wools & bamboo's, silks, fleece....each has it's use & place
I generally use cotton or wool fabrics-although i've incorporated some silks & specialty fabrics into a few quilts and often use a polyester (minkie or fleece) backing on special request quilts
I often piece with cotton thread- but i have a long arm & use mainly polyester thread for long-arm machine quilting---cotton tends to break easily with the highspeed of the machine....so, you see- each has it's place & use- there are no rules---use what you like-but don't be 'closed=minded' & stuck in a rut---be willing to try new things & don't (pooh-pooh) at others for using something different from your preference.