I like both. My experience is limited but I will throw in my two cents anyway

Note: I'm considering Buck US made slipjoints vs Case US made slipjoint.
From what I have seen, the Buck 300 series are very good knives. Solid, well built, made with a good stainless steel, and overall offer alot of value for their price. They are "working" knives, so handle materials are not really fancy. Their "weak" points, to me, are the limited selection of models, and the grind on their blades (I'm talking about the current production models). Their F&F seems consistent enough.
Case offer many more patterns, in alot of handle materials, providing a good quality knife as well. For unknown reasons, some models seems way more consistent in F&F than others. Their stainless steel is not that bad but I still think that they could offer something better at that price point (like Buck or Opinel, to name a couple). Their CV is better, but it's not offered for all models (and - incredibly - some of their bestsellers are not offered in CV).
Both brands keep their pulls quite manageable (I hate stiff backsprings), which I appreciate alot.

As a general idea, Case knives seem to target buyers who prefer fancier knives (or collectors), while Buck's slipjoints apparently aim at "pure users".
Both have their place in history and market
Fausto
