My experience in C3 was that early UU's were more effective. In the early game, there are fewer troops, and they're of lower quality, so small increases in power are more meaningful. Later on, when there are hordes of units trampling everything, and they're more powerful, gaining a small increase in attack doesn't mean as much.
Truthfully, though, I've found that the UUs which had mobility bonuses were much better than those with combat stat bonuses.