for me, i prefer to use the wooden pencil one.
i notice when i use the mechanical pencil compass,either lead broke
or loosening of the tip occurs.
if u wanna continue using the mechanical-pencil de, dun exert too
much strength on the sharp tip.
u can try this...when marking out arcs, draw ur arc lightly first,
then go over it a few good times to make it darker.
theres no fear of putting a tad too much pressure to ur compass,
resulting in alterations of compass angles or tip loosening.
some teachers are against this idea as it might result in 2 or more
arcs being formed instead juz one accurate one. but i always like
to challenge them...if ur good, ur good.
theres no need to worry abt double arcs drawn if u hold on to ur
compass well.. 
