Following from the EMPF HelpLine, prepared by Pat Kane:

In my limited experience with spray fluxing, the biggest advantage seems to 
be the lower amount of material used during run cycles.  The working life of 
the flux is greatly reduced in foam fluxing due to moisture absorbtion. 
 This is not seen with spray since the flux is maintained in the container 
until sprayed.  Cleaning of the spray machine is a tradeoff with the 
foamers, they both need to be cleaned well after each use to insure maximum 
performance.  An added bonus to spray also seems to be less dripping of flux 
into the preheat area of the wave solder due to reduced material volume 
being used.  This is not to say that the coverage results are inadequate. 
 As far as quality issues, the proper setup of the spray should result in 
acceptable results.  The hole penetration of the flux will be a setup 
parameter that will be of the greatest concern.  Hope this helps.