I'll be honest, there is not much different for day to day use. For example, using a tile to access facebook, email, twitter (with apps) all work the same(some little changes in email but, not much for every day use). Now you get the nice look and cusomization of having the 3 tile sizes, that was the biggest change.
if you have a device running 7.8 there is no major reason to upgrade to WP8 right now. Why ?
1. Zune sync is WAY better than the so called WIndows Phone app (it's a POS app right now)
2. FM radio support, Hardware is there on WP8 but, no software to use it
3. Day to day use is the same with 7.8 and 8 (live tiles)
4. besides nicer display, there are no games that really need a dual core CPU if any apps right now.
If you use your phone for email, facebook, twitter and phone calls, I dont see any MAJOR upgrade YET to going to a WP8 device over a 7.8 device. The 7.8 upgrade is nice, new tiles, Bing for wall paper plus a few other minor things.
Also (if you have the right phone) Wp 7.5/7.8 devices are hacked fully, so you can get access to a ton of cool apps that a factory phone wont have or even the apps appear in the marketplace. There are no unlocks besides Dev available for Windows Phone 8 yet (I really enjoyed this aspect of WP7).
I used the 7.8 update for at least 2 months(beta version then final)on my HTC Trophy before going to a HTC 8x Windows Phone 8 device and I honestly dont notice much in day to day use....
If your still on contract and ending soon, get 7.8 and wait till WP 8.5/9 devices hit, you'll thank yourself later....