We have used many rolls of both. We haven't seen any noticeable difference in them. I think the siser weeds a little easier and I also like that its available in 150' rolls. So that is what we use. You can also use foil from either place, or any screen printing foils from anywhere for the most part.
We have done long term wash testing on both and they both hold up very well.
We are local to stahls and love all their HTV products, so you cant go wrong either way.