Here we go again!
50-series vs. 80-series. It was one long and detailed thread SX-1250 vs. SX-1280 in AK. The overwhelming consensus was that:
a) SX-1250 better built
b) SX-1280 has some unobtainable parts
c) opinion on look SX-1250 vs. SX-1250 was about 50:50
However, the original question was about value. And this is a different animal.
I don't have these monsters, but I have SX-750 and SX-780. Here is my thoughts.
a) SX-750 is MUCH HEAVIER, like maybe 32 lb vs. 25 lb (and 5wpc more power). SX-780 feels like scaled down unit.
b) SX-750 is rarer then SX-780 according to numbers of eBay listings. For instance, right now eBay selling only three SX-750 and fifteen SX-780 (5 times more).
c) both are about the same in the look department; I prefer SX-750.
However, eBay shows that generally SX-780 sell for more then SX-750. I have no idea why. Possibly because 780 is higher then 750 and uneducated buyers see it as 'upgrade' unit. That's the only reasonable explanation I can think of.