Considering none of my games ever get to the Marion Reforms, I consider the Hestati and the Pricsiples (bleh, I hate spelling these things) to be quite effective ground troops for Romans at 170d/turn. They both have the same attack, but the Pricsidnwuknfgeubgs (henseforth called Hestati v2.0) have slightly higher defense at the same per-turn cost. Considering that most towns you conquer in Gaul (important to note if you play the Julii) are 6000 population or less, you will be churning out a lot of Hestati. Once you hit 6000 and grab the updated barracks, switch to Hestati v2.0 since the actual Hestati will be obsolete by that time. However, both versions can easily hold their own against phalanx (oddly enough) because of their ability to outmanouver an enemy, especially in sieging on-top of city walls where the spears cannot be used in phalanx formation.
Triarii have too high a requirement (12000 population barracks) to be really cost and time effective, therefor I consider the Hestati and Hestati v2.0 to be better.