Citation policies: original papers or recent monographs?
Why not cite both?  


  The following theorem is due to Doe [37]; see also [28] for a modern exposition and further background.


Here [37] might be Doe's original paper, and [28] the recent monograph.

Math journals rarely have page limits, so there's no disadvantage to including more citations.  This way you credit the original author and also point the reader to the resource you think will be most helpful.