I've found that for general workflow, they're about the same, except that Mercurial has a slower checkout process and doesn't resolve conflicts as nicely. I'm not sure if there are features it has that git is missing..
Or if you're really ambitious (also: independently wealthy), try a new niche each day. Must have been draining coming up with a new toy-size web project to do daily...
Working for a company that has started adopting JSON Schema as our primary validation tool, I can definitely agree that XML still has the upper hand when it comes to a seamless experience in data validation. Schema validation is still really primitive (though with the love JSON has been getting in the past couple years, it shouldn't be too much longer until it's in fighting shape).
Something about XML just has never really sat well with me - whereas JSON just works when it comes to actually modelling objects in a way that makes sense and can be picked up quickly. I can't say I've yet to find a place where I absolutely needed a feature of XML that I couldn't find in JSON, but would love to know more situations where I can be proven wrong.
I think ultimately if the goal is just to JSONify the world, then that's no good - but if we're looking to make protocols more accessible and hackable, JSON may be the way to go.