The lower lactose content of WPI is the superior protein powder for lactose intolerance. The majority of intolerant individuals can use WPI without experiencing any digestive symptoms.
WPC contains more milk fat than WPI with around 4-7% fat. This is not necessarily a bad thing, but if you are counting calories or want more control on fat intake WPI is for you.
Whey Protein Isolate digests quicker than WPC.