Substance dualism and property dualism are two positions in the philosophy of mind, and they’re trying to answer questions like “Hey what’s the relationship between the mental and the physical?” or “what’s up with consciousness?"
According to substance dualism, mental things and physical things are two totally different kinds of things. It would probably mean that no matter how much we study the brain, even if we managed to achieve a perfect understanding of the brain’s crazy intricate inner workings, it won’t be enough to understand the mind, or our mental experiences of subjective consciousness, because we’re looking at the wrong fucking thing. "Hey, quit studying that brain, it’s not even the right goddamn substance, you idiot.” Maybe if we figured out how the brain and the mind hook up, we’d learn something about the mind, but if substance dualism is true, then no amount of information about the brain would ever be enough to fully explain how the mind works.
According to property dualism, though, there’s only one kind of substance, and it just has different kinds of properties. So we only have to look at one thing, the brain, and that’s where we’d find both mental properties and physical properties. If we fully understood the brain, we’d have enough information to also explain but also, it would mean there’s a bunch more shit to understand about the brain than if substance dualism is true.
So, in short: substance dualism says “Oh no you’ve got the wrong thing entirely, stupid” and property dualism says “yeah, no, go on, keep looking at the brain, we’ll get it eventually."
That seems simple enough – let’s just keep looking at the brain and if we get to the end and we still don’t understand the mind, we’re done, right? But here’s the problem: how do we know when we’re at the end?
Like let’s say we could go to God and say "Hey, God, we think we’ve figured this brain shit out but we still don’t know the deal with the mind. Can you check our work?” God would read what we’ve got so far and then He might say one of two things.
If substance dualism is true he might say, “Oh, yeah, this is pretty much all right, you’ve got the brain figured out, but you’ve been looking in the wrong place the whole time you idiots."
If property dualism is true, though, he might say "Oh, no, see, you got all the physical parts of the brain, sure, but there’s extra shit in there you have to explain, like how consciousness arises. Yeah, sorry dude, you have more work to do on the brain after all."
The tradeoffs between substance dualism and property dualism are actually really common tradeoffs in metaphysics. A lot of times when we have two competing theories, they compete in a specific way: one has simpler, more obvious explanations but also a lot more shit in it, and the other one only requires like one or two things to make it work but the explanations are banana-pants complicated. Sometimes we call these two kinds of theories abundant and sparse respectively.
And you can see why this might happen, right? If we could explain the entire universe using photons, we would. Why complicate things? But we can’t explain the entire universe just in terms of photons, so we have to add things. So when it comes to how many things, property dualism gets to be like "Ew, why the fuck do you have an entire separate kind of thing? Where does it come from? What does it even look like? That’s so extravagant and unnecessary, what’re you, new money? Is your granddad even rich?” and substance dualism has to be like “Hey shut up so what if it’s totally invisible and we utterly lack a description of how it might work."
On the other hand, when it comes to explaining what they’re trying to explain (in this case, something like ‘the nature of consciousness’), Substance dualism gets to just say "this is easy, the mental is just a whole 'nother fucking thing, that’s the explanation” while property dualism has to be all “What if the way consciousness comes from the brain is [RESPECTED BUT CONTROVERSIAL BOOK LENGTH EXPLANATION]."
This is as always oversimplifying things but what it comes down to is, when we eliminate shit from our theories, we have to make the other shit explain more – we have to somehow explain everything that we used to have extra shit for. Sometimes those explanations are really simple and elegant, like when Copernicus went "Oh look, just put the sun in the middle?” and sometimes it seems bizarre and improbable, like where Dan Dennett is like “oh consciousness is just a bunch of metaphors I guess” and it’s like hmm. Usually we’re somewhere in the middle: which theory you’d prefer is a function of just how many things you’re willing to tolerate vs. how hand-wavy you think an explanation is.
I should say that if I’m not mistaken, most contemporary debates aren’t even between property dualists and substance dualists anymore, they’re mostly between property dualists and some form of eliminativists, who want to go a step further and eliminate the idea of even separate kinds of properties. Those motherfuckers want to take candy away from all the babies, which means they have a ton of explaining to do and not a lot of resources to do it with.
So it goes.