I have both of these tools. The mechanical pencil I have the fons and porter one but it is the same thing.
Mechanical, makes a very fine line. I used this tool to draw the dragon and tiger on my MC quilt.
http://www.quiltingboard.com/t-161213-1.htm
As you can see the design is quite detailed and fine so no problems doing fine lines with this tool. The ceramic lead rubs off very easy. I had to remark my lines numerous times during the quilting process. If you want it for DSM quilting, I have my doubts the marks would hold up with all the rubbing the quilt gets with manipulating it through the throat. If you quilt on a rack, I think they would hold up ok.
The Bohin product makes a much fatter line. You can see the marks clearly on this quilt I was working on and used this tool to mark the spines of the feather in the border,
http://www.quiltingboard.com/t-138359-1.htm
the chalk does not hold a point at all. It works well for my purposes though, which is usually just basic registration lines. Additionally, other users of this product have posted problems removing some of the colored chalks, specifically the orange. I have not run into that myself.
Edited to add, for your purpose of marking the fabric for piecing the mechanical pencil will be fine. It comes white leads and graphite leads for marking light fabs. Sorry I missed that the first time I read your post. I would not buy the Bohin product for the purpose you indicated.