One distinctive difference is that heap can be used as a measure of amount while pile can only measure amount based upon the composition of the pile. Eg., "A heap of loving is good for a marriage." "Their marriage is good because they piled on the hugs and kisses." With both words, the usage is commonly idiomatic. Each is used in many clichés. Football:Notre Dame is piling on the score. Iowa needs a heap of scoring if they are going to catch up to Minnesota in the second half.