The Facts (In Plain English)
First things are first: Chimera is actually just a different version of Chameleon. The underlying code in the two bootloaders is almost the exact same.
This code is organized by an Apache Subversion system (abbreviated as SVN) into two parts: the "trunk", and its "branches".
The branches are versions of Chameleon that test out new features, while the trunk is the official (and most stable) version of Chameleon, which all of the branches originate from. Unfortunately, while the SVN system used by Chameleon is great for developing new features in its branches, the official "trunk" version of Chameleon is still slow to update. Plus, the whole "trunk" and "branch" system of organization is really confusing for normal users.
However, the experimental branches of Chameleon are actually pretty stable themselves. In reality, the best feature of Chimera is that it's more accessible than the other branches. The official homepage for Chameleon hasn't been updated in three years. The developer page for Chameleon is completely unfriendly for non-developers. And the only way to find an actual standalone installer for various branches of Chameleon is through 3rd-party sources like OSx86.net (where you'll be faced with dozens of different versions of Chameleon available for download).
Meanwhile, Chimera is well-integrated into the tonymacx86 ecosystem. There is only one version of Chimera available for download, in tonymacx86's official download page. No confusion.
Which One Should You Use?
Here's a hint: it doesn't matter.
The most popular branches of Chameleon update every few days. Chimera updates every few weeks. You're probably not going to notice the difference. In fact, once you get your Hackintosh up and running, you won't have to update your bootloader ever again, whether or not you're running Chameleon or Chimera.
Just install whichever one is more easily available to you. People following tonymacx86's methods should install Chimera, since it's bundled with Multibeast. People following alternative methods should install Chameleon, since it's bundled with tools like myHack and Kakewalk.
There are always exceptions. Despite their intertwining development cycles, Chimera still supports a slightly different set of graphics cards than Chameleon. Of course, this is only an issue if you use a graphics card model that isn't particularly popular in the Hackintoshing community. If that's the case, you might have to install Chimera instead of Chameleon, or the other way around. Once that's settled, you can simply move on. And don't forget to have fun Hackintoshing.