I don't really see the point in comparing them. They're different devices for different markets.
The Switch is for people who want to play first party Nintendo titles. That's really the only reason for its existence. Without Nintendo's first party lineup, the Switch would be just another Arm based handheld, and a fairly unremarkable one at that.
The Switch is all about exclusivity, the Steam Deck is the exact opposite. Not only is the Steam client, and the massive library of games that it gives gamers access to, available on scores of x86 devices and hardware configurations, the Steam Deck operating system will soon be available pre-installed on multiple, third party devices, and it will be available for anymore to download and install on any device they want.
They're not just different devices, they're vastly different company philosophies.