I recently took a quilt class from Kimberly Einmo. I loved her. She said if you use cotton it will sink down into the fabric better when you press it to set the seam.
I use cones from CT for most things. I have some Aurifil (cone) that I use for quilts where I need to be really precise. They are both 50wt but I see a big difference.
I have some serger thread and when I pull on it, it will not break. I sometimes use that on charity quilts for the quilting. My rule is if you tug on it and it breaks---toss that thread.