Which one is more valuable in a postdoc application? a full paper in a B* conference or a short paper in a A* one?
A short paper at an A+ conference is better on your CV than a full paper at a B+ conference, because readers will pay attention to venues and will most likely ignore paper lengths (even if you include them -- which you probably won't, because they take up room). 

But, it seems like you're trying to decide upon a venue on the basis of only the immediate impact on your CV, you should take a broader, longer-term perspective.



Backstory: The OP's paper was originally ten pages and it has been accepted at the A+ conference on the condition that it is reduced to six pages.