Pretty sure you can convert S Corp into C Corp at a later date and declare the multiple classes of stock needed to raise money. I believe if you convert into a C Corp from the S Corp, you have to wait a period of time (5 years?) to convert back into an S corp if you so wished. LLC is a different beast. I've personally never been involved with an LLC.

I'm not an attorney or tax guy, just a startup guy.

Update: Brad Feld Explains this well/confirms some things. http://www.feld.com/wp/archives/2006/02/s-corps-vs-llcs.html