I have both. Received the ST version in bundles that I purchased. From a functionality perspective they are pretty much the same providing motion and temperature readings and both made by Centralite. But they differ significantly in size and price. The Iris version is much smaller and cheaper. I personally will be buying more Iris motions as I need them.
There is lots of discussion in this thread about them:
There is a custom device type mentioned, but I had success following the instructions in Post 502 to utilize the SmartThings device type which makes this device run locally on a v2 hub.