I remembered that AndroidPolice did a very
thorough Mobile Security App Showdown
- but that was in November, 2011. A lot has changed since then, so I went searching for a more recent comparison.
Here's one I found from August 2013.
(Hint: Cerberus wins)
Cerberus has a lot of advanced options that can make it harder for a thief to bypass, but I still come back to Android Device Manager. It's free, it's included and integrated with Android (so no additional running service to get in the way), and it's tied to my Google account (so not additional credentials to keep up with). Some of the others can easily best it in features, but I don't think any of them can touch it for simplicity and ease of use. There's even a handy mobile client
in case you want to keep tabs on your phone from your tablet (or the other way around).