neither? Harrisson's I've always considered a reference type book. I imagine one would use it a lot more during their clinical years, but never for step 1.
Use it along side you're 2nd year courses, but thats not really a board prep book by any stretch of my imagination.
Cecil's is a good book, but its an internal med book.
I can't see either explaining some of the details step 1 would test you on.
Think you might be better served by reading Robbins and correspond the subject to a section in Cecil's.
But come board prep time, leave those books behind.