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one half of me says xp because a lot of companies and businesses are shunning vista at the moment, so they are still using xp, but then on the other hand, vista has just come out and it'd be an advantage to have the latest in mcp!
The long answer is that most of your support will be XP now, so you should do XP first, but as the amount of Vista you need to support increases, you'll need Vista certification as well.
I'm currently trying to motivate myself to write the Vista exams. I just hate the bloody OS so much that I feel like I'm certing up on MS BOB, for crying out loud.