Convert used road bike or MTB for touring? I would like to convert a bike to a tourer and I have access to a load of frames. 

I've seen people saying that mountain bikes convert well to tourers and haven't heard much on road bikes. This is confusing because road bike geometry looks waaaay closer to touring bikes.

My gut instinct is to convert a road bike since the geometry looks closer. What is better to convert?

Edit: My route is the west cost of the USA from Vancouver to San Diego. I am going to be fully loaded, a.k.a. fully self sufficient but I am investing in lighter gear.
Old steel hybrids work pretty well for converting to a touring application. Like others have said the ideal build depends on how much your going to carry and the distances you plan on going. Obviously comfort is a big factor as well. Fenders are necessary if you plan on doing any distance at all really. 

With that said yes, I would lean toward a road style over MTB but I would stick more with an "urban" geometry rather than a full out road bike. I have a fondness for old steel frames for these conversions as they take a beating and come with a bit of nostalgia.

As for components, like others have said fenders, good tires with puncture protection, a comfy saddle, rear rack or panniers.. it all depends on how deep you want to dive into it. I would recommend starting small, do a day trek or an out and back weekend and see what's sore afterwards, that will gauge your bike as well as your commitment to it and let you know where you stand before you go emptying your wallet.