Coats & Clark was the very first thread I used when I started sewing. It's fine for basic garment sewing, but I had a hard time getting the colors I wanted so I typically use Gutermann for garment/home dec projects. It does a decent job with sewing straight lines. C&C in particular starts to struggle when you add in curves, don't backstitch to reinforce seams or put much strain on it. It is more prone to snap & will definitely shed like crazy in your machine. Gutermann sheds pretty badly also, so that means stopping to clean out bobbin case & "floss" my thread run. And it's not as durable in the wash. Plus, you definitely do not want to use either of those for quilting -- especially FMQ -- because the stress of changing direction & going through the thick batting will substantially weaken the thread. In addition to all that, the dyes used in the thread can bleed or fade with heat and/or washing.
According to the Connecting Threads website, MasterPiece is a 2-ply thread. Superior is definitely a well-regarded brand and long-staple cotton truly makes a difference. It's also Egyptian cotton, which is a finer, stronger cotton than other varieties of cotton.
Their Essentials threads I saw online say they are mercerized. Mercerized cotton is great for hand-work, not so great for piecing by machine (and would be almost impossible to machine quilt with it). It is a chemical treatment that makes the thread thick & crisp. I used it once before I knew better & it gummed up my machine & gave me terrible results. I use it for hand stitching on my binding & that's about it. I use bee's wax + Aurifil for hand quilting because I didn't really even like it for that. It's just too, too stiff.
Personally, I like working with Aurifil 50/2 for piecing & 40/2 for quilting (I buy at
http://www.hawthornethreads.com/threads/aurifil). I feel spoiled every time I quilt with it. Aurifil threads are also Egyptian long-staple cotton and come in hundreds of colors. The dyes hold up well in the wash & with the pressing required to prepare the quilt top. It definitely costs more, but the money I save by not having to service my machine as often helps to offset the additional cost. However, you would definitely do well by switching to Superior cotton. It's not my personal favorite, but it's a very, very nice thread.