I typed too fast...actually the dual stems are so you can moniter the tire pressure via one valve as you fill, empty with another.

google the nitrogen thing...widely discussed online (I stick to plain old compressed air)

as for 18 vs 19 in terms of looks and performance it all comes down to the stance of the car, the bodykit, the look you want, etc etc. Most of the wheel and tire combos out there at the end of the day have a very similar weight in 18 vs 19 - for a pure track car, it might make a difference, but even then, I suspect the difference is far less that people claim. On the street, if you can tell a difference in how the 18's accelerate vs the 19's (assuming you are using tires of the same rolling diameter), then an engineering job should be in your future

 

Nitrogen molecules are larger than oxygen molecules, which helps retain the gas in the tire. Due to how the oxygen molecule is made up, it seeps through the tire sidewall slowly over time. Also, nitrogen has much more stability in pressure than air does when temperatures fluctuate. Lastly, oxygen can cause oxidation on the insides of your tire, which = rust. You will never have this problem with nitrogen. Overall if have aftermarket rims, it would be highly beneficial to add nitrogen if you can find it. You'll only find it in upscale wheel & tire retailers, so look for a place that specializes in brabus, lorenzer, etc. Hope this clears things up for you.