They're both excellent lights, and either one would serve you well. Perhaps if you share a little bit more about how you'd like to use the light (e.g. on your keychain, as an every day carry light, as a backup, etc.) we can help you weigh the pros and cons for each role.
For what it's worth, both are phenomenally bomb-proof lights made by well-respected CPF members who use high-grade materials. You can't go wrong with either of them. If your focus is size and simplicity, the Aeon's a smaller light with just two levels. (Twist for low, twist a little more for high.) If it's flexibility you're after, the Ra offers four different pre-programmed levels (including a phenomenal ultra-low), and the ability to program each level to your liking. You may also want to consider the Ra Twisty...
Good luck making your choice! I have both, and love `em to death. But as Armed Forces explained above, they're very different lights...
- FITP