I've been playing with thread the last few nights. My supplier of Aurifil is going out of business so I ordered a bunch of thread including weights I haven't yet used (28wt and 12wt).
As Scissor Queen mentioned - the smaller the number, the heavier the thread. 12wt is a pretty thick thread that I would only consider using for quilting when I want the quilting to be very noticable. Ditto for the 28wt. I'm playing with different bobbin thread weights and tensions to see what I like best for these two thread weights.
For piecing, I generally stick to 50wt or 60wt if it's a particularly thin/fine fabric (some batics for example).
Generally when I quilt with 50wt I will use a 60wt in the bobbin. I
I've tried some poly threads and I personally don't like them. I don't like the way they behave, I don't like the way they look, and I don't like the way they way feel. This is just personal preference though - there is no reason in the world you can't use poly threads to piece or quilt cotton - and lots of people swear by Superior Threads "Bottom Line" thread for the bobbin and I believe that's a cotton/poly blend (I think it's a 60wt).
Seriously ... it's best to buy (or borrow if you can!) a smorgasbord of threads and try them out in different applications (on test pieces - NOT on your quilt). Make notes. I keep sticky labels in my thread box with notes on the tension I used, the bobbin thread I used with it, the needle I used, and how it all behaved with each other. This way if I decide that a 12wt would be perfect I know how to make it work.