Which one is more valuable in a postdoc application? a full paper in a B* conference or a short paper in a A* one? Assume the hiring committee (Computer science) are reviewing the CVs of two Ph.D. graduate's with almost equal skills/expertise. 

Regarding their best publications, one has 1-2 short papers (e.g. 6 pages) in A* conferences (top-tier) whereas the other one has 1-2 full papers in B* conferences (second-tier) in comparison, and both of them have an equal amount of other publications in lower ranked venues and journals.

Then which one can have better chances in getting hired as a post-doc researcher?
Consider
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.