It is time to ultra-necro post :D   
      
   With todays knowledge about science-fiction and both of the Star Trek and the   
   B5 universe, this can be answered easily:   
   The shadows would win.   
      
   Let's take a look at the specs:   
      
   First of all, the Borg cannot assimilate the Shadows. The borg were not able   
   to assimilate Species 8472, because of their immune system and their organic   
   ships.   
   The shadows also use organic ships, which unlike the Species 8472 ships are a   
   sentient life form on their own who don't even need a pilot and the species   
   itself is much more agile than a Borg Drone, plus, they can camouflage   
   themselves, so they can    
   attack unseen (this was seen when the Shadows killed the Vorlon Kosh Naranek).   
      
   Secondly the Shadows don't rely on conventional laser or phaser or whatever   
   light weapon, but on particle beams, which work as the following: critical   
   high density mass is accelerated just under light speed and shot at the   
   target. Their slicer beams were    
   powerful enough to slice massive spacecrafts in half life a katana cutting a   
   leaf. There is no weapon in the whole Star Trek universe, which obliterates a   
   ship with such ease.   
   There is no logic method to defend against this with an energy shield, as the   
   weapon first of all is way too fast and secondly the damage is focused on a   
   single spot (which would require ridiculous ammounts of energy to counter).   
      
   On top of that, the Shadows have planet killers, which can be used against   
   whole fleets. Within the vicinity of this planet killer arrays ship engines   
   suffer under interferences rendering them very ineffective (only the Excalibur   
   class dreadnaught was    
   hardly capable of moving due to it's Vorlon technology, which is extremely   
   advanced and made to counter the Shadows).   
      
   Also each and every ship (even the planet killer) can instantly blend into   
   hyperspace, making them intangible to the Borg attacks (which have no access   
   to hyperspace).   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)