I mean, the difference is an interesting value proposition. Photoshop CC, if it functions the way SaaS is supposed to, is now all the new iterations of Photoshop. It isn't like buying CS6 vs CS7, where they're mostly compatible with each other forever, because in two years CS6 will be less compatible with Photoshop CC than it is today, and in 10 years Photoshop CC will still be what you're continuously paying for, but CS6 will be mostly out the door if not entirely obsolete.
So the question is, do you want to pay for something that you may have to switch up from now, or would you rather pay the same amount to use CC? The adobe master collection was a couple thousand dollars every two years, and Photoshop CS6 is something like 700 bucks if I recall correctly.
So if you want to do the breakdown, consider how long you'll be using Photoshop CS6 for. Photoshop CC + Lightroom CC is 10 USD a month. Photoshop CS6 is 700 USD flat, and CS6 extended is 1000. How many years will you use CS6 for before you choose to (or have to) move to Photoshop CC? If it's more than 8 years, get CS6 extended. If it's between 6 and 8 years, get CS6. If it's less than 6 years, just get Photoshop CC. CC costs 120 USD a year, but in order to beat that value you have to use CS6 enough years that its cost over the lifetime it's used for is less than the cost of CC per year. CS6 for 5 years is 140 USD a year, and for 6 years is 116.667 USD a year, so if you use it for less than 5 years and 10 months, you're paying more than you would using Photoshop CC.
TL;DR: If you're gonna use CS6 for more than 6 years, buy it. Otherwise it's cheaper to use Photoshop CC.