Apple makes developers wait to release new versions (approval process) while Android does not. As a result, iOS is a better choice for A/B testing since it has longer cycles of deployment.
A/B testing has nothing to do with the application review process. It's for either A – bringing a new service online that cannot handle the load right away (server tuning), or B – testing the UX and fine tuning it before releasing it to entire user base (client tuning).