I havent hear either of these on a YFZ but a buddy of mine has a white bros E-series and likes it alot. He said it runs as good as either of his old pipes plus he can quiet it down in about five minutes for running around the house. If you get the E-series you will want to buy some more discs though. They dont give you enough.
The E2 series pipes are nice too but haven't seen any in person but the idea looks nice. Real easy to go from mild to wild.