I always play with ranging barbs for all civs.
Resource settings: I never use standard resource; in some games I use Legendary and in others Strategic Balance.
Hum, playing the Celts, you will start in forest (assuming that start biases weren't turned off); when combined with the 2 faith for free per turn from turn 0, I would be building a Scout first and have the pantheon on turn 5. In the mean time, I would hope my Warrior hits a upgrade ruin to upgrade to the UU to start faith-farming the barbs. Playing them, I might wait a bit longer before building a worker.
Playing Ethiopia, their religious advantage doesn't start until they build their UB, so I would build that first. In contrast to playing the Celts, I would hope my Warrior does NOT hit an upgrade ruin (I don't like regular spearman and prefer the normal upgrade to Sword). Playing them, I build the worker at my usual time.
I'm a fan of Tradition tree, and would pretty much use the same buy settlers for 500 gold + the 2 city NC. An aggressive AI is equally likely to interfere with plans; when that happens my early game combat moves to within my own lands for a bit with the normal use city bombard & archers to kill most enemy units, only using melee units to finish off where they can do so safely.
I find somewhat faster founding times with the Celts but faster time getting the enhancement with Ethiopia as the Celt UA starts degrading around the time the religion is founded from needing to improve the forest next to it. (This hurts the Celts more in Legendary than in strategic balance.) And I also get more faith post enhancement with Ethiopia.
This is going to give the religious edge to Ethiopia; that slightly slower to religion as them compared to Celts is still likely to be first to a religion unless either the Mayas or the Celts themselves were in the game. Meanwhile, the Celts are more likely to be a spot or two later enhancing the religion than Ethiopia.
Happiness: Celts have the happiness edge with their UB replacement for Opera House.
Culture: Ethiopia has a culture edge with their UB.
Units: Celts suffers from their UU being on a "bad" base unit. In fact the type of unit I wouldn't generally consider fighting the regular AI with but would only use against barbs. It upgrades to a unit that is acceptable against a small attack, but not suited for the big wave of AI units where you need to kill them via bombard from safe tiles. Half of the UU also goes away when upgraded to Pike. But the Ethiopian UU suffers from chances are really high that the UU attribute might as well not exist. (While I might be fighting within 4 tiles of my capital early on; I'm not going to be by then). This makes Ethiopian UU even worse than the Celts.
Initial wave of AI attack: Ethiopia is likely but not guaranteed to have a combat edge going my normal tradition start. Now if they end up conquering their nearest neighbor during it, their edge is lost against several civs for the rest of the game. (But not all, quite a few AIs go very wide). But if I've done so well dealing that wave to have conquered my nearest neighbor, I've effectively already won.
Overall, this appears to make Ethiopia the winner, at least with my preferred playing style, but neither civ belongs on the "worst civ overall" list.
