Quest for the royal dragon egg
 

Does anyone know if there is an upside to helping the griffins steal the royal ice dragon egg? Do you encounter the griffins later on?

The immediate consequences are:
Helping the griffins: Both shops gone, you fight two dragon heroes (second fight optional).
Helping the dragons: only one shop gone, you fight the griffins, additional scrolls and an item as reward, followup quest which will reward a Dragon Slayer's Sword (not sure if this is fixed, and it is probably almost useless after this point. But it's shiny).

I can't say for sure as I sort of skipped a bit at the end.

I'd say all in all you are better off helping the dragons and that the griffins don't offer any upside (as far as I can see).