I'd have to lean havily toward the Romans at higher difficulty levels. Three reasons:
1) Hoplite and Legionnaire both cost 1 citizen to hurry. At that stage in the game, most all of your military should be hurried out, and the Legionnaire is clearly superior for the price.
2) You have to be very very militaristic throughout the game. You get noticeably more Great Leaders when you're militaristic.
3) Scientific is all but useless at the higher difficulties. You don't research techs--you broker them. Research should stay down at 0% until after you've established a clear scientific dominance over everybody else. So you don't even need Libraries/Universities for a long time. By the time you do, you're so filthy rich from brokering you can just hurry the Libraries out for gold, scientific or no scientific.
Too bad the GOTM is Greeks. Bleh. I guess that means earlier Libraries for the culture and you get to broker the free techs you get. Yay.