I am fairly new to using these DISigns, but can say that I have used 50% cotton, 50% poly to make shirts for my family and have been pleased with the results. I do not have the experience of LOTS of shirts to compare, tho.
I bought 100% cotton shirts yesterday at Academy for $4.00 and they had many colors. Also, Hobby Lobby had their 50/50 shirts on sale last week for $2 (like Michael's this week). It seems the 50/50's can regularly be found for $2, whereas 100% cotton may be harder.
I wish I had more experience to share with you. I do not know if the transfers last longer and better on cotton shirts. I just know that they look just as good (IMO) on either. Hopefully someone with more experience will post.
I just wanted to add something since no one had posted yet...