My preference is to upgrade to Europe, rather than back, if I can upgrade only one way. Two reasons: first, I want to sleep on the flight over so I enjoy my vacation more, or get more work done, when I'm in Europe. (Melatonin also helps.) Second, I like the extra attention from ground and FAs when I start my trip. It's like starting my vacation a little early.
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"If you want to win the game, write the rules."