Two music recommender websites, Pandora (a content-based recommender) and Last.fm (a rating-based social recommender), were compared side-by-side in a within-subject user study involving 64 participants. The main objective was to investigate users’ initial adoption of recommender technology and their subjective perception of the respective systems. Results show that a simple interface design, the requirement of less initial effort, and the quality of recommended items (accuracy, novelty and enjoyability) are some of the key design features that such websites rely on to break the initial entrance barrier in becoming a popular website.