The general promise of employing the motivational power of games for serious purposes, such as performing physiotherapy exercises, is well-established. However, game user research discusses both the approach of gamification, i.e. adding game-elements on to a task-focused application and of serious games, i.e. injecting task-focused elements into a more fully-fledged game. There is a surprising lack of empirical work that contrasts both approaches. We present both a casually gamified application and a serious game with purpose-driven mechanics that provide different frontends to the same underlying digital health application. This application aims at supporting physiotherapy sessions for chronic lower-back afflictions. Results from an explorative pre-study contrasting both approaches indicate a clear preference for the serious game version, capturing higher perceived motivational components (autonomy and relatedness), as well as higher immersion and flow relative to the gamified version.