well the difference will be much in the same way it feels with AL as opposed to Carbon fiber forks. most people agree that AL lets all the vibrations of the road go right though the bike into the rider, and with carbon fiber this is slighly less. (also a weight saving issue here) personally I think there will be a slight difference in the feel between the AL seatpost and the Carbon fiber, and most of the hype is in the weight saving.

I have not heard of American classic either, but since you are a new rider I would not worry about it, get used to the bike and riding before you think about upgrading any parts. Another good idea is to go to the bike shop and see if they will let you test ride a bike with an carbon seat post. you will know for sure then
happy riding