Definitely, Oximeters are gonna win.
The reason we should choose is obvious as pulse oximeters are the dedicated device that rapidly detects changes in blood oxygen levels. Since they are a dedicated device and designed for medical use, one should always rely on the pulse oximeter readings.
Having said that, it doesn’t mean smartwatches don’t show the actual readings and are not reliable. They show accurate readings and are handy when you are outside or where oximeters are not with you. But, even smartwatches manufacturers don’t suggest it for medical use and just for fitness.
So, better opt for oximeters and for daily use or for fitness purposes; use wearables. Thanks!