Best way to tackle bcrypt

In your opinion, what's the best way to tackle bcrypt with a cost of 12 (ie $2a$12$...)?
While I think using the GPU would the best choice when we deal with dictionary + rules attacks, what about when you try something easier?
For example bruteforce for digits only passwords or very short ones?
Is the GPU still the best tool or in such cases we should fallback on CPU?

Any thoughts on that?