Isn't Sony's (or Samsung's) custom controller's party trick allowing the GPU to read directly from the SSD. Sure I read that somewhere. One things for sure, MS cheaped out on the SSD and controller and Sony didn't.
If PS5 is the dev platform of choice this gen (and it seems to be according to the GDC questionnaire), then games will use the SSD and other tricks it employs, this is an absolute fact. Then you will see the other platform running ever so slightly worse, maybe a touch longer for textures to pop in during game play in open world games etc. It won't be much, but it will be there for all to see.
Obviously, it the multi platform dev platform is the Xbox, then what you are saying could be true, but it will still load quicker.
Truth is we still have no 100% idea what the PS5 is doing. But your statement that Ms cheaped out on the SSD isn't exactly fair, and I will stick up for MS here. The SSD MS will use isn't exactly some shit cheap thing, and if Sony didn't go the way they have done, people wouldn't be saying as such. The Ms solution is still a great one, just not "as great" as Sony.
As for the multiplatform stuff, more than likely Ps5 will be lead platform, yeah,
which makes sense. But this difference isn't like, for example, a car that runs 60mph or 120mph There is a cap in which things start to show diminishing returns. And the truth is that, at least for the next few years, until assets get much, much better and larger, you won't have much of any perceivable difference to the end user.
When you say texture pop in, yes that's true. HOWEVER, this is the part that people get a little confused on so allow me to explain. They visualise the pop in issues we see in this generation, for example classic unreal engine streaming issue. So people are instantly making the connection the Xbox SSD will do this, and the ps5 will be instant. The truth is, there will still be pop in, and there will still be delays. You will never eliminate this because you can't overflow the memory stack with that much data at once. So, get used to it. What we CAN do, is limit it. But here is the issue... Simply having a bog standard SSD will already do this to the point that now, we are looking at VERY minimal differences. So before with a mechanical drive on old DDR memory we had a delay, and now, we have a near instant but still visible delay.
So throwing bullshit figures out there that aren't realistic but work for visual aid, if it took 10 seconds to populate the world with the highest mips and best quality LODs, it will now take 1 second with a bog standard SSD (which the Xbox doesn't have as it seems to be pushing higher random read speeds). The PS5, will be loading those assets at 0.75 seconds (because it doesn't directly correlate to twice as fast as mentioned previously).
So in other words you are going from 10 seconds for this generation to 1 second for SX and 0.75 for PS5.
Again totally bullshit figures but accurate to a sense of putting it into a perspective people can see.
So, that amazing expensive top of the line balls to the wall SSD has now not only cost a shit ton to produce and create, but it's only saving you 0.25 seconds. Which, when it comes down to it, is barely anything.
Now let's flip it again in favour of Ps5.
The times this WILL come in handy is when you start to use random memory consumption but at higher sizes and far greater frequency (or to put it in layman's terms, the textures get higher res and there are more of them). So now you're able to load far more data faster, and essentially this can make a difference. But this is something you won't see for a good few years, and as such won't present itself for a long time.
I've been working with asset optimisation for a LONG time, and know the impacts it has on memory, and know what help an SSD can have when used correctly. But sadly, the Ps5 SSD is being hyped up as this great saviour that will destroy everything in its path, when the truth of it is that due to the increased speeds both systems will have, the actual perceivable improvement between the two next gen machines will be minimal, to the point that the average user (read: not a digital foundry viewer) won't ever even notice.