Well, either way, here's my opinion.
While I do really like Bizenghast, I know how it is to want to do a new project after focusing on one thing for so long. So, if you were to take a break and focus on a new series, I think that would be perfectly understandable. In fact, I've done the same thing in my writing numerous times (without finishing a story, to my chagrin...).
I would just really hope you could return to tell more of Bizenghast after you had completed the new story. Plus, there's the added bonus of more experience and fresh eyes to make it even more epic than it already would be.
So, if it were me, I would wrap on the current project and move on to the new one, then come back to Bizenghast at a later date.
Just my two cents.