The short answer
Same maker, similar clip, very different promises. The Hero MD is a certified Class IIb medical device built to detect breathing movement, with a vibration at 15 seconds and an alarm at 20. The Pico 2 adds an app with sleep position and skin temperature, is rechargeable rather than running on a replaceable battery, and can be used up to six years old, but Snuza sells it as a child-care aid and says it doesn't monitor breathing.
Owner reviews are also warmer for the Hero MD, while complaints about false alarms are more common for the Pico 2, whose Bluetooth app needs your phone nearby.
My call: the Hero MD, easily. It costs less, and it's certified for the job most parents buy a clip-on for.