They're one in the same.
Now.... the actual SSD itself may not be as fast as some of the premium PCIe NVMe offerings. Do you know the make model of the stock SSD? If it's not absolute trash, you're unlikely to notice the difference in an upgrade though (unless your usage is quite advanced).
I generally don't trust that I'm going to get a decent SSD in much any laptop (they also overcharge), so I generally order the model with the smallest SSD (or none at all) with the intent of installing my own. A decent 500GB NVMe SSD costs around $110 on sale these days.