Greek units tend to be the toughest for their type - a Greek Hoplite is better than an Egyptian Spearman or a Norse Ulfsark - but also the slowest and most expensive. A Spearman travels at 5 meters/second, but a Hopite travels at only 4.2. A Spearman also costs only 70 resources while a Hoplite costs 120.
The Greeks, along with the Atlanteans have the only way to upgrade their titans.
Dionysus's Bacchanalia upgrade gives all myth units, including titans, 5% more hp. On a titan, that gives it a total of 7350 hp, as compared to the base 7000 hp.
Also, Hera's Monsterous Rage upgrade gives all myth units, including titans, 25% more attack of all types. On a titan, that gives it a total of 88 crush attack, as compared to the base 70 crush attack.