Is there really a difference? A pantheistic God is not only within everything, it IS everything. An omnipresent God is only within everything. Of course, if God is within all of me, then isn’t all of me God?
I feel that the problem is similar to how Jesus and God are supposed to be separate but the same. If Jesus had a separate nature from God, then would he still be God? How about if Jesus had the same, shared nature but a separate will? How could Jesus be God without God being Jesus? Yet we are asked to understand that some part of Jesus was not God.
The only real difference is the concept of free will. In a pantheistic God, God does not have a free will. We do, but he is a part of us or, rather, we are a part of him just as all matter in the universe is a part of him. Removing God’s free will sounds ridiculous.
If we have free will, how could God not have free will? That would almost make us superior to God in a way. So we have developed this distinction between the omnipresent and the pantheistic God. God is within us, in some compartment reserved for supernatural beings most likely located in our heart. In this way, he can retain his free will and we continue to have ours. But can we still qualify this God-passenger as omnipresent?
If God is omnipresent, is he within our thoughts? Is our thought itself God? It’s a vast, gray area which makes me wonder if a pantheistic God is the only form of God which could truly be labeled as omnipresent. If I have free will apart from God, then God does not exist in my free will, and that would negate his omnipresence.