The difference is that if the device comes with Steam OS, then it's ready to go out of the box and you're assured the hardware has good Linux support.

If it's originally a Windows device then you may have to jump through additional hoops to get everything working. Also you'll have to deal with allowing other OS's in the BIOS if it's locked.

Also you've paid Microsoft for a license you won't use.

The flip side is that there's work to make a native Steam OS build for 3rd party portable devices:

https://arstechnica.com/gadgets/2023/06/the-linux-coders-turning-the-rog-ally-and-other-handhelds-into-steam-deck-clones/

and

https://chimeraos.org/