In general, I feel server side is
far superior for developers and client side with a WYSIWYG UI is a fabulous option for other stakeholders. I could talk on this subject for a while but my plane is about to take off. Suffice it to say you can't match power, flexibility, and speed of server side with client side testing. You can move away from just twiddling text to actually redoing workflows or business logic.
I didn't write ABingo with a pain point included, but for Appointment Reminder it will be helpful to do AB testing in non-Rails marketing pages, too. I wouldn't append too much effort optimizing your solution for the needs of people who have written AB test frameworks though.