Depends on your goals. If you buliking WPC is fine, it's about 80% protein and slightly slower in digestion than WPI, but lots cheaper and does the trick.
If you are on a carb restrited diet or are counting calries as part of a fat loss diet then WPI may be thego.
I use WPC pretty much all year round, I don't see the need to spend the extra cash on WPI, I prefer to spend it on free form aminos.