Are there really any substantial differences? I've found that many of them can be circumstantial to the game itself--i.e. ANY AI tribe that starts on a large continent with plenty of elbow room and good terrain will become strong or even formidable if unchecked early enough. Since I am generally a late-bloomer militarily myself (i.e. I don't build BIG militaries until my shield output is big (i.e. around industrialization), but when I do the balance is tipped), I generally don't check the far-off big civs--but ones on my doorstep get nipped in the bud pretty early....
Anyway, I've seen the Romans very strong with lots of cities and a stiff tech competitor, and I've seen them pathetic prey of second-rate civs--all depends on the circumstances. Ditto with Aztecs, Russians, Germans, Japanese, Sioux, etc.
That having been said, the programmers, through personality modifiers, have tried to make it so you can expect different things from different civs--i.e. IF the Russians, Greeks or Mongols get strong, they are more likely to be an aggressive threat to you if you don't act upon it--whereas strong Babylonians, Indians, Germans or Romans won't threaten you as much militarily as in the tech arena.
However, as for the "rational" rating, don't count on that! I've probably been sneak-attacked just as much by those "rational" statesmen as by the aggressive ones--maybe even moreso, it seems--even though "rational" in game terms is supposed to mean relatively trustworthy....