What benefits are there to side with Sabal or Amita?

It doesn't really matter thus far. Later on in the game you still can decide if you want to do Sabal's or Amita's plan. I switched a couple times so far and either way I had a negative reaction (eg. cities being attacked and people die). It seems so far, that the main plot doesn't change and those decisions only decide on what the prologue or epilogue mission looks like. IAmita is more focusing on economics and Sabal on saving the culture of Kyrat.

SABAL EPILOGUE

While Ajay might have been fighting to keep Kyrat from turning into a forced-labor drug state, it would seem that putting his faith in Sabal's "traditional" beliefs may have been a regrettable choice.
After the fall of Pagan Min, Sabal's leadership becomes nothing short of religious fanatacism. He forces Bhadra into the position of Tarun Matara and publicly executes anyone who stood in the way of his quest for power.

AMITA'S EPILOGUE

Despite Ajay's desire to see a more progressive and modernized Kyrat, it seems as though Amita is more concerned with preserving her empire than the actual well being of the Kyrati people. After the fall of Pagan Min, Amita's Golden Path begins forcing villagers into labor in factories or drug fields and forcibly drafting children into their armed forces. As Sabal says in his parting words to Ajay, Kyrat has become "A land of equal opportunity where everyone's a slave."

WARNING: SPOILER ALERT

If you side with Sabal I'm not sure, but if you decide to side with Amita you end up recovering some intel that seems not importat at first, but that is later used to prevent a major attack from the enemy.