The best approach would give the user the same experience, no matter what device he's using. Building 2 seperate sites hardly ever reaches that goal, most mobile sites are limited/cripled versions of the regular site. At least let the user switch back on demand.

What does 'mobile' mean anyway? A low resolition screen? Limited bandwidth? Touch screen? It's silly that a 1280x900 notebook with dail-up modem is considered not mobile, while a full hd tablet on wifi is considered mobile.

Just make your skin/site responsive, thus work with the relevant capabilities (resolution, cookies, javascript, flash, etc) of the device in question. Don't make separate versions or at least allways ask yoursel why would I "deny a visitor access to this or that?"
e.g.: Don't provide low quality graphics because you detected Android or IOS and decided this must be 'mobile' (whatever that migth imply), but provide low quality graphics when you detect slow network speeds and/or a low resolution.

As long as people keep creating 'mobile' versions of websites, a phone or tablet will never be able to replace a desktop PC, even when their capabilities far exceed those of a PC. And we all know how annoying it is that you have to start up your PC just because some site decides you may not acces the information you know is there (or doesn't allow you to log in) using your phone, because it's 'mobile'.