I'm told the NISMO product is made by AEM, buy it from the source and save some $. You can probably spec's on the K&N website...I just bought the Airaid intake myself and I'm satisfied w/ it and that's probably the most in-expensive one of the bunch. It sells for around $180.00.
I think the key to being satisfied with this mod is to keep your expectations realistic. The intake alone won't make your X go like a raped ape. It will help the MPG and your throttle response. Some folks report increases in HP, but I didn't seen that in mine.