The reason I ask is what are the (true) advantages of starting developing the mobile version of a site first and then going up on screen sizes. In my work I've experienced that we always have a lot of troubles with the mobile versions of the sites we develop specially because we never have a design to follow and we always spend a lot of time on it, I know we should have a pre-made design but the 99% of the time we don't get one. So is starting for the hardest and/or longest time consuming part a better idea than starting for the easy and relatively fast part?
I'm using the standard way for the site I'm working on now. The main reason is that desktop and tablets are the majority of visitors. Mobile is up and coming (and catered for, don't worry ;) ), but it made more sense to start from desktop in this particular case.
Other than that I would say that mobile first is better, mainly because it will focus you on what has priority on certain page. And from there it's easy to move into multiple columns when you have the room :)
Design shouldn't be done in Photoshop anymore, at least no full comps, more like styletil.es. Designing with code is much easier to do in my opinion.
So in your case, I'd say go mobile first, design it out there and adapt the site for the various widths with media queries.
Homer Gaines, CPACC
Interactive Designer + Front-end Developer + Certified Accessibility Professional + Speaker
Mobile first because, if the content is planned correctly, it's easier to add to a design than to take away.