I look for long cotton staples or polyester. Mostly I'm concerned with weight. I use 50 wt Aurifil-cotton or 60wt Bottom lline-poly for piecing. The fine size of there threads allows the seam folds to lie flatter.
As to quilt stitching, this depends upon density of stitching and the look of the stitching. Usually I use 30 wt poly wrapped poly-Perma Core. I have used 12 wt for high stitching visibility. If it's a high density, I like Bottom Line.
The choice also depends upon the machine I'm using, some threads work better on each of my machines. I have several collections: serger thread, 4 cones of each color, cross wound for the Brother, straight wind for my Bernina), LA thread (due to the speed of that machine I want low lent and strong thread).
Haven't heard of Marathon before. Where do you get it? Do they have a web site?