Style Sheets v. Mobile Sites
Mobile Sites > Recommendations
If you have an existing site: Do what you can to optimize the site you have using simple fixes suggested by the W3C's Mobile Web Best Practices (discussed in The "Zen" Factor) and using mobile specific style sheets.
If you are building a new site: Try to balance the needs of desktop and mobile users when structuring your site, but keep in mind who your primary audience is. Follow the W3C's Mobile Web Best Practices as closely as possible and use mobile specific style sheets.
If after having optimized your site for mobile content you find that you have a very significant amount of traffic from mobile devices: Consider building a mobile site.
OR
Be a rebel. Build a mobile site first, and then adapt it for desktop platforms. You can read more about this approach in The "Zen" Factor.