I'm about to order K&N from 4wheelonline.com but I have two options-- the aluminum and plastic tubing. So, I would to hear from you guys about your personal experience with this air intake. Which of these tubing is better in terms of performance and quality? I'm not expecting significant hp or mpg improvement; just hoping to improve the sound of my Xterra. Thanks for any help.

The Intake-
I might suggest a Volant Intake. While both improve volume of air that flows, the K+N draws hot air from the engine compartment, while the Volant box extends and aligns with the original cutout leading to the space between the engine bay and the outer front quarter panel above the wheel well. If anything, there is a chance that colder air would come from this area.
As far as aluminum or plastic... How do you want it to look. I had a 'K+N' type with rubber hose connecting two aluminum pieces for 5+ years. Functioned just fine. May not have been an actual K+N. It took some work to get everything to line up and synched down with the tube clamps, and the black 'Heat Shield' was a nuisance more than it helped (IMO), but once it was on for a while it was manageable to work with when needed. The Volant I have on now is a solid molded plastic tube to a filter into a solid box enclosure (which aligns with the manufactures cutout to draw from above the wheel well). Solid black is my preference, easier to clean and maintain its look.

TBS
If the sound is the goal, an Airaid Throttle Body Spacer adds a nice 'turbo' like hiss to the uptake around 2K to ~3.5k RPM. www.airaid.com It was kind of a pain to get on my 04.

My 2 Cents
I've tried to do things here and there to improve gas milage and as a byproduct I have worked with both styles of intake piping and the T.B. Spacer. Go Volant for some performance increase and TBS for sound.

Fuel Econ
As for fuel economy, I've found performance greatly relies on how you drive. I've improved 15.5-15.8 mpg in 06-07 to right around 17.2 now while commuting. Tempering my accelerations and cruising at 45 or below 55 when possible, (which works in my area), coasting in neutral on down hills and approaching red lights. I have yet to do a drive greater than a tank at one time to get a solid mpg estimate on highway milage. But these "habits" as well as some assistance from aftermarket parts have helped me.
Prob more than what you were asking for. Hope it helps.
-Cheers

Thanks for the reply. That's a lot of info.

I heard good things about K&N; that's why I'm planning to give it a try. Though I think the aluminum tubing will heat up very quickly than the plastic one. About the Volant, I know some guys have this CAI with Powercore filter, and looks like they are happy with it. So I might consider it as well if I found a good deal.

From all I've read from xterra owners around the internet, Volant is the only way to go.