Antimony [1] is stable and user-friendly. You can get started without knowing about the underpinnings, just by connecting predefined shape nodes into a graph – basic modeling is possible with just the mouse, as shapes can define handles in the UI. Using Python for scripts is also a bit more accessible than Scheme. The downside is that hacking raw shapes is less pleasant, because they're defined as strings implementing a DSL. Developing on Antimony is also slower – much of the code is UI and glue to attach the geometry + graph engines.
Ao is more experimental and unstable. It's much lower-impedance for folks that want to work directly with f-reps, but doesn't have a click-and-drag UI with which to get started. The geometry kernel is also newer than Antimony's, and has slightly better performance. From a development perspective, I can play more easily with new algorithms without a cumbersome UI.
For folks that aren't as comfortable with coding, Antimony is the right choice. For f-rep development, Ao is the right choice. Otherwise, it's a matter of personal preference.