As JoelM stated, butting is simply making the tubing thicker (stronger) where it's needed (usually on the ends). Internally butted means that the outer diameter of the tube stays that same, while the internal diameter changes (which is what we typically see). Externally butted means that the internal diameter of the tubing remains constant, while the outer diameter changes. I would say that the biggest difference between the two is superficial, but they will have (slightly) different structural/ride properties. Internally and externally butted basically means that GF has attempted to maximize their strength gains and their weight losses all in one frame.
I would imagine that Platinum Seriess (SS) and Platinum Series (Xcal) are pretty close to the same allow (if not exactly the same). It is simply a proprietary tubing spec just for GF based on a ZR9000 aluminum alloy.
I think mech vs. hydro is a personal thing. I like my mechanicals, because they stop me just fine, and are really easy to set up and maintain (no oil, no bleeding, etc.), but it's really a personal preference. Each type has its advantages and disadvantages.