My 2cents:
Equal army sizes from the peaks of both, hard to say. I would amagine that Rome would win. Their flexibility and lack of "chivalry" (rules of war, etc) would probably give them the edge.
1 on 1, a Spartan would win. Nearly any soldier could bet an equivelant Roman. Romans were deadly in groups. Celts were deadly one on one. Greeks were best in groups, but were trained for both.