Uuuuuuhggggg....
Ok, I am an American. But there is a problem with the premise "Which side" and "America.."
Here is the problem. Define what is meant by "America". Because as far as the American people, there is diversity in opinion on this question, there are also many Americans who are very ignorant regarding what is Sunni and Shia. Once there was a time when Americans did not have a good or bad opinion regarding Islam, in fact many had a "Lawrence of Arabia" romantic vision of Islam and some were interested even in becoming Muslim, surprisingly more than is imagined and certainly more than today who might want to go join ISIS which is a tiny, tiny lot indeed.
But that was then, not now, and things started to change in the 1970s with Islamic terrorism, until today I would say the majority of Americans today hate the Islamic extremists and terrorists and have suspicions about Islam. In the past most Americans never thought about Islam much if at all beyond the romantic visions of Islam I already explained. But after so many terrorist attacks, hearing "Death to America" 24x7, yes the extremists finally got the lazy attention of Americans, and actually it was pretty stupid of the jihadees. Because, they have the attention of Americans now, and Americans can be very violent indeed, many are natural born killers and cowboys and indians, if they start coming to really kill you they will, first will come smoke, then the locusts will spray death guns into the smoke, killers jump out of death machines into the smoke as machine CIRCLE around and will kill left, right, then retreat because next will come the fire and burning. After Custer's last stand they adopted the Apache Way. Others do not understand that. Old Southern white men with gray hair look at maps, but on the field the real orders come over the air in America Indian languages no one can understand but those on the channels, then the locusts move.
Real dumb, the Islamic terrorists. Actually, Islam could have been a natural partner with many Americans. Think of the Right-Wing - the same Hollywood that the Right hates is the same Hollywood the Islamic world hates for trying to turn their sons into junkies and their daughters into whores.
But the Islamic terrorists attacked the Right-Wing. Real stupid.
So what "America" will do, as far as taking sides - it all depends, it can change at any moment. Like I said, I do not speak for whatever this "America" is that is referenced in the OP. But I think I can clarify the HERE AND NOW which can change tomorrow. Even the current government is largely aligned with what is my opinion, and which is a good part of this "America" you ask about. But keep in mind, there is no one opinion in America, no way.
For one, there was an allusiun to "Saudi Sunni" verse (non-Saudi?) Shia. NO. Wrong. Wrong, not the American view.
Right away, I project "Saudi" as perhaps the OP meaning the Royal Family. I have a big, big problem with Sunni ISIS mass murderers. THEY ARE NOT THE SAUDI I THINK OF, and certainly NOT the Saudi Royalty and it doesn't matter if the Saudi Family is Sunni, these ISIS nutballs want to KILL the Saudi Royal Family, and I never support that.
Also, these ISIS lunatic cult is KILLING innocent Shia. I have a BIG problem with Iran trying to get the nuclear bomb, but... I support the Shia innocents being mass murdered by ISIS. So it is not surprising also, Iran wants to protect their Shia faithful. Yes Iran sponsors terrorism, but so did Stalin in WW2 but we worked with Stalin to help defeat the Nazis who were like ISIS. So I am ready, very carefully, with verification, to work with Iran.
So that is my view. Not all of America, but not a minority. We are now looking at ISIS - and American soldiers are coming. This is NOT America taking "sides" in any Sunni Shia war even though that may be a reality.
It is about other things.
One day America and Islam will be partners who can work together. As odd or strange as this sounds to you or others, you are now seeing the beginnings of this happen, and the world becoming even smaller still. Islamic terrorism, the jihadees will be defeated.
Time will prove that.
