Specifically the form factor. I have a lot of nostalgia for the GBC and SP, and the smaller size means it’s much easier to carry and whip out while traveling. Currently, my Steam Deck is used for when I’m on a long (6+ hours) flight or when I’m staying somewhere for a weekend. I don’t feel great about bringing it out while taking a bus or train, and it’s a bit much to bring out while sitting at an airport gate for 45 minutes.
That being said, I would hate to split all my saves and progress between the two systems, so maybe I’ll just remain on the Deck.