Insoluble fiber is the stuff that keeps you "going". It's roughage. You need it. Soluble fiber is digested, sort of like a starch would be. I hope that helps! Let me know if you want more clarification.
Both will have some effect but the insoluble stuff provides the bulk. $725 MILLION per year are spent on laxatives (in the U.S.) - so it's probably good that we're having this conversation. Fiber is your friend! :)