For me the only difference between the two is that the Principes are better...

I place them on fire at will and sometimes guard mode as well (but the jury is still out on the effectiveness of guard mode) and I march then up to the enemy close enough so when they stop they throw their pilum, then when they are done throwing all their pilum simply get them to attack... The trick is to get your line close enough to throw but not so close they provke a counter charge...

If I need to force them to engage (in an emergency) I was use Alt to force a melee attack so they don't mess around with the pilum and get stuck in straight away.