CSS media queries are awesome, but as you stated if you want to use geolocation and such, you should really look into modernizer js library which can help you detect what browsers actually support it. Using modernizer you can use geolocation for compatible phones/browsers and then use a fallback for uncompatible browsers, (Older browsers like IE < 8) and older phones.
Make sure though that you code in a way that if the user cant use geoloation, they can still view the content in another means. You said that your website starts in a month. Unless your VERY familiar with geolocation and combining it with an API like bing maps or google, I suggest doing media queries for elements that don't rely on scripting (Such as geooation, web storage/localstorage, and etc..) until you fully test it on your localhost web server on your computer to get all the kinks out and to make sure you fully test for all browsers and phones.
Thats just my opinion though, but remember, older phones and browsers still surf the web. I suggest media queries for now if your just going to make it phone compitable from your whole site. If you plan later on to make an actual mobile version of your site like using redirects to have mobile phones to directed to a subdomain like mobile.yoursite.com, then I would look into geolocation and localstorage. Unless your site already has it incorporated into it, I wouldnt add it. Again, unless you make a fully mobile-only version of your site.
Hope this helps!!