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I’m not sure what the situation is like where you are, but one way to gauge current demand is simply to look at the job listing in local trade newspapers or magazines and compare the number of positions for Salesforce developers and Hadoop developers. Frankly, you are likely to do well with either skill set, but Salesforce is going to place more limits on your options. Keep in mind, Salesforce also uses Hadoop, so even in your binary choice of the two, you can see that Hadoop has wider use. If I personally was choosing one with the goal of maximizing my career options, it would be Hadoop.
Here are a couple of relevant links you might find of interest: