The short answer
These two answer different questions. The Snuza Hero MD asks one: is my baby still making breathing movements? The Owlet Dream Sock measures pulse and blood oxygen and sends alerts to a base station and your phone. Both are certified medical devices, but only the Snuza is the type used in The Lullaby Trust's CONI programme, which leaves out oximetry monitors because the extra data can add to anxiety.
The Owlet wins on convenience: alerts reach you in another room, it keeps working as your baby rolls, and the app is excellent. The Snuza wins on simplicity and cost: around £95 against £280 or more, nothing to charge, no Wi-Fi, and no connection problems. Its big weakness is that the alarm sounds on your baby, so you need to be within earshot.
My call: if you room-share, as safer sleep advice recommends for the first six months, the Snuza Hero MD covers the core job for a third of the price. Choose the Owlet if you specifically want alerts on your phone and you're confident the numbers will calm you rather than keep you up.