There is no difference whatsoever between Spinoza and the best Advaita. He says that there is only one without a second, pure existence-awareness, and mind and matter are two of its modes of appearance. He calls pure-awareness God, and says everything is appearance in this pure-awareness. This pure awareness is pure existence, pure bliss, pure knowing, and not limited in any way. All appearances are limitations, or what he calls finite. The human consciousness is really just the thought, 'I am this body-mind' arising in this pure awareness. There really is no actually consciousness or you apart from pure awareness, and you are that. The human identity is the various thoughts and perceptions as a group that hang off the body-mind thought passing in awareness, as he discusses in Ethics. It is quite simply the cleanest, most logical exposition of non-dualism you'll ever see. Actually many popular teachers set up a human ego trying to 'rest in awareness' or 'see one's true nature' and thus preserve a subtle dualism. In his Ethics, Spinoza makes no such self-contradictory mistake. Pure non-dualism, as he says, 'There is only one substance and no other, and all things, matter, thoughts, are appearances or modes of this one substance.'
In some ways he's more correct than traditional teaching, as they often appeal to the human phenomenological experience of being as proof of pure-awareness, which is suggesting that human experience is some sort of actual existant or has some validity, which it really doesn't. He merely demonstrates how dualism is self-contradictory and illogical to the mind, without having to define the non-dual, appeal or bolster the human ego and so on. In fact, the word 'non-dual' is really a denial of the dual, not defining the supposed nondual, which would be some sort of human concept always. Teaching should, like Spinoza's, never deviate from merely rejecting the dual, not making weird appeals to human subjective consciousness and they telling you how to get rid of it - that's all kind of insane if you think about it.
Spinoza's quite sane. Quite rational. Leaves Nisargadatta, Buddha, Ramana and most others in the proverbial dust of their concepts and ego.