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black looks more formal. like if you are going into nice places and such, i would get black. you said you are backpacking, so if you are going to be off of the beaten path and living a bit more rustic, i would get brown. it looks more rugged and less formal
$0.20 for a ziplock bag to keep your passport and other important stuff in one place, not lose it, AND keep it dry? What a deal! Plus, depending upon what type of backpacking you are doing, weight may be an issue. If you mean backpacking in the sense that you are carrying a backpack with your possessions while riding buses and staying in hotels and hostels, this will not be as much of a concern.