There are a lot of differences in both the cases
But in the end Dharma is the seed for any/all of Rama's decisions.
Please think along the following lines:
Case 1: Vali and Sugreeva -
Vali and Sugreeva are own brothers.
Vali did ill things to Sugreeva in an unprovoked manner.
Vali was not any stranger to Sugreeva (which is the case with Ravana and Rama). He lusted for his own brother's wife.
And not just that - a brother also snatching all the material, emotional and social belongings of his own brother is in itself a crime.
So, the water had reached well above the nose - and hence it was a clear decision to make there.
Case 2- On Ravana and Rama
Ravana was a stranger, equally established, devotee and powerful emperor as was Rama.
Ravana might have been provoked by what was done to his sister.
Ravana was a mighty king himself and hence Rama spared him a chance to get educated...
And before sending Angada - Hanuma was sent to assess the strength of Ravana.
And according to Hanuma - Ravana has not touched Sita at all - and he was willing to wait till Sita accepts him, by all means.
Had Rama approached Vali - he could have got Sita very easily, as Ravana got defeated by Vali.
But Rama chose Dharma and hence chose Sugreeva for help.