So, my prediction (FWIW) is that virtually every 70D buyer will opt for the upgrade to the 75. Why do i think so? because my causal observation was that the same thing seemed to happen when the 85KWH battery model had the option for 5 KWH to be the 90. In my trips to the Service Center/supercharger/delivery center near me, it looked as if nearly all the Model S cars coming in during that period were 90s, not 85s (and a surprising number were Ludicrous as well).
Could be wrong. Maybe the dynamic is different for those buying at the low end of the range versus the upper end....