Sounds like a combination of production issues and underestimation of demand are going to make the 6+ harder to come by at launch. According to the WSJ are making 2-3 times as many 6's as 6+'s right now. While I think in the long run the 6 will be more popular, at launch (especially because the 6+ sold out preorders so quick) most standby people are going to be gunning for a 6+, especially the hardcore people who line up the day before. What limited stock is available is going to go fast. I think there's still some hope, but I would get in line around 4, if not sooner.
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That's exactly what Apple did. The first people to come (aka the people who preordered) are going to be the first served. Everyone who preordered stayed up late to make sure they were the first to preorder.