A comparison between the
Big Three of quads you listed is a bit problematic for me, as I have only tasted the Westy and St Bernardus beers once (not aged at all). For me, nothing beats a well-aged Rochefort 10.
I'd say St Bernardus is the best bang for your buck and works the best when you "just want a (strong) beer". It doesn't really compare with the creamy mouthfeel of the Westy or Roche. Obviously the taste of the Westy is marvelous and wins the St Bernardus and a fresh Rochefort by a landslide (very complex plum/raisin/blackcurrant notes), but that doesn't justify the price at all. An aged Rochefort 10 tops the Westy, therefore I can't appreciate the price difference of nearly 20 Euros.
Out of all times, with these prices I would probably pick up the Rochefort 10 for pure enjoyment and St Bernardus 12 for more of a refreshment. If I could only get a hold of all three beers, age them and do a blind test... Also, I still want to experience the Westvleteren straight from the monastery draft.
Sorry for the disorganized message - I should really get some sleep now.