The advantage of the SmartThing sensor is the added sensor suite. You have temp, vibrations, movement, and tilt. This makes it great for things like detecting is a washing machine has stopped running, or for use on a Garage Door opener, or for remote temp sensing, as well as the “default” option of open/close detection.
The advantage of the AEON sensor is that it’s Z-wave. (The SmartThings is Zigbee). This has two advantages: First better compatibility with systems other than SmartThings. If you decide to change systems later the AEON should work with ANY system that handles normal Z-wave products. There’s a far less likely chance that a system, even if it has a Zigbee radio, will be compatible with a proprietary sensor like the SmartThings multi-sensor. Second, many other common home automation plug in products (like light switches, outlets, Thermostats, plug in modules) act as Z-wave repeaters meaning you’ll have a better chance of getting a solid signal through to your hub if you’re a long ways away. Zigbee repeaters are a lot less common (The SmartSense motion sensor when plugged in does act as a Zigbee repeater).
So… in short: If you’re thinking that you may be changing systems in the next year or so, or if you are worried that your location of the sensor will be too far from the Hub for good communication, then I’d recommend getting a Z-wave open/close (like the Aeon item). Otherwise the added sensors and options for the SmartThings device make it more desirable in my opinion.
Either would work for using the magnetic open/close part. Just like a normal door/window, you mount it to the door so that the magnet in the small part lines up with the magnetic sensor in the large part when the door is closed. However, this can be tricky to do on a garage door given that the inside of a garage does not always provide good mounting locations to the two different parts.
The SmartThings sensor, though, can be used in ‘tilt’ mode. If you change the device type on the website (support is able to help you with this is you’re unsure of how to do this), then the module reports open or closed based on the angle at which it is silted. When it’s vertical, it says closed. When it’s horizontal, it says it’s open. Now you can just mount the device anywhere on the inside of your garage door… you don’t even have to use the magnet side at all.
Personally I have both devices and I like the SmartSense sensors more. All for somewhat minor reasons. The both reliably do what I expect them to do (tell me that a door/window is open); however the SmartSense, in my opinion, looks slightly better (doesn’t attract as much attention as the glossy bubbly AL one). In addition like @chrisb said, it also acts as a temp, tilt, and acceleration sensors. While I currently don’t have any use for the acceleration sensor, if I ever intend to repurpose this sensor, I can easily do so.