Well, I ended up loving Pepsi Throwback, but I didn't end up changing my mind about Mountain Dew Throwback. The flavor that I've grow to love as the "nectar of life" is not the same with sugar. There is a different after taste, reminding me of diet MD. There seems to be less bubbly goodness and it's missing the distinct Mountain Dew taste. It reminds me more of the generic versions (Mt. Breeze, etc.).
I'm actually hopeful that Pepsi Throwback sticks around. And if PepsiCo decided to start selling all their sodas with sugar instead of HFCS, I'd still support the MD franchise. But as far as flavor comparisons go, I'm actually going to have to say the HFCS version does win in this taste test.