It depends on your goals and lifestyle.
Membership is better suited for those who like a steady schedule and a well-disciplined routine. If you are highly motivated and with a clear target goal. you can gain optimum results. Ideally, attending 60-70% of the classes provides you with faster results in all aspects of yoga. Most importantly, with a membership you have access to all available classes, replays and special videos in "On-demand" library.
Class packs are best for individuals with an intense lifestyle. If you are interested in various yoga practices or curious to try different styles. I am a multi-discipline yoga teacher who shares a vast range of knowledge. Thus, class packs are the best way for you to choose specific topics.