I have used poly serger thread (verigated) when I wanted the stitching to show. Even the Mettler #50 is a bit heavier looking than #50 mercerized cotton. My experience is this with this type of thread: pre-wash your fabrics for minimal shrinkage/distortion, use a smaller needle with a bigger eye (embroidery needles are usually easier to find, but sometimes I get lucky and find quilting needles- and stock up when I do- or "sharps" as a last result- NEVER "universal" because they cut or puncture instead of sliding between the fibers of the fabric). This reduces the thread pop-ups you will occasionally see when machine quilting. I loosened the top tension just slightly, and left the bottom tension alone. (Because of the smaller needle size you play with the tension a bit less.) One word of warning about this thread: it is a poly thread which means it is stronger than your cotton fabric, so I (would recommend) only use it on large quilts where I am doing larger quilting designs. (If I decide to add smaller quilting to the quilt, I switch to a cotton thread.) After using this type of thread you will want to wash the quilt, throw it in the dryer for a short time, removing before completely dry, and then spread out to dry, "blocking" it before binding if possible. I am no expert, but I do love to play with many threads just to see which effect I like for each quilt I make. If you want to stick with all cotton thread I can recommend Valdani #40, or Star (I think is the name) #50. Both quilt a bit heavier looking and are available in many colors. (The Star brand is more effective if you use the same color in the top & bobbin.) You can also "Trick" the eye by using a #40 or #50 for your main quilting and then do your background quilting with a bit finer thread like a #60 on up to a #100.
With all of the new threads in the market and access to the internet the possibilities are endless; jump in & have fun with as many of them as possible! Here is one rule I learned to always be true- keep like threads in both top & bobbin (cot/cot, poly/poly, however, you can mix silk & rayon). Poly thread will cut cotton thread.
Sorry to be too verbal on this subject, but it is so near & dear to my heart!! I could go on, and on, and on... Have fun!! Have fun!! Have fun!!