Mmmmh, that's a tough choice !
Granted, Celts have a more interesting background than the Vikings IMO. On the other hand, Vikings are a culture that players will relate to more easily.
Now about my (would be) setting...
Unfortunately I have not much time to devote to its creation, and since I changed of ideas so many times, I hardly got anywhere. Nonetheless, the setting is inspired by the early dark ages (the end of the Roman empire), as well as the medieval Byzance when it had a Varangian guard (Viking mercenaries). Then, on top of that add a Conan-Hyboria flair, plus crawling Cthuloid cults trying to subvert the decadent empire and its official Stoicism+Christian-inspired religion (Church of the Logos). As a side note, no elves-halflings-gnomes, etc. Only new races, with a grim / hyborian flair.
There is a slight advantage to using Norse/Vikings, as the players would grasp the setting more easily. On the other hand, Celts would be called Keltoi, be inspired from Slaine's celts (rather than the historical ones), and live in the Highlands, thus be called highlander barbarians (who wear sorts of kilts) by the civilized people.
I don't want both Vikings and Celts, as the setting doesn't revolve around these "barbarians", but revolves around a Cthulhu-like demonology from ancient ruined empires to the east.