You should never limit yourself to only cotton thread for piecing or quilting! My normal piecing combo is Aurifil 2 ply 50 weight cotton on top and Superior Bottom line 60 wt polyester in the bobbin. BTW Superior does sell bottom line in prewounds but I wind my own. I use neutral colors in both, usually a grayish green color that seems to blend with everything. If I am piecing dark, I switch over to my dark neutral which is a charcoal gray. If piecing light to dark or bright to dark, I stick with my grayish green neutral.
For quilting I prefer poly. I love glide which is a 40 wt trilobal polyester with lovely shine. For longarm quilting I definitely prefer the prewound magnaglide bobbins. I also use super fine poly called Invisifil which is 100wt and looks and acts like silk without the expensive silk price tag. I save my real silk for applique.