You've probably finished the sign by now but I thought I would offer my 2 cents.
First of all, besides being an oxmoron, translucent black (and bronze) is completely opaque. If you put a 2 mil cast black next to translucent black on a lighted sign face, you will see that the cast is somewhat blochy. The translucent will be dead solid opaque. While this may not make much difference from far away, it will be noticable if the face is viewed from close range. The other advantage you gain using the translucent black is consistency in the matte finish of translucent vinyls.
If quality of your work is important, then "translucent" black is the way to go.