its easier for a fan to suck air through a filter rather than push air through, so to answer your question... its better to have the fan after the filter.
regardless of what the fan is supposed to do all carbon filters are designed to have air drawn through them so have the fan after the filter
I have always read pull through a filter even though I have seen more than handful of people around here pushing air through one. If you are looking for some general ideas on filters and how to map our your circuits you can check out my signature <-- Damn I have been pimping my filter a lot lately