Unless you're under pressure to submit a score, once per week is a much better decision.

One cannot cram for the GMAT and hope to achieve one's highest potential result. It's much better to take a full week to absorb each lesson and rigorously practice the methods that you've learned.

The above applies no matter which course you take.

I agree...once per week is a much better pace. You'll need to do homework for the class and fit that in around regular activities and work. Plus it takes time to practice each week and get to a comfort zone so that you're getting the most out of the class.