I'd suggest that you spend some time on the superiorthreads.com website. There is real good info about needles and thread.
I use 50/3ply cotton thread for both piecing and quilting (for the most part). The only difference is that I use neutrals (beige, cream, grey....) for the piecing and matching colors for the quilting.
Masterpiece Thread (from Superior) has a 50/2ply option for piecing which means that the thread is a hair thinner and it will help with that 1/4" seam. They also have a 50/3ply Masterpiece now.
Some renowned quilters use Bottom Line in the bobbin for quilting and I am planning to try that soon. It is a 60 weight and makes the quilt stitches disappear into the fabric.
I am also planning to use a 40 weight for quilting which provides a slightly heavier look.