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Both have been around a long time (relative to internet forums) and are great resources. Club Lexus is a bit larger and caters to all Lexus models instead of one model.
My.IS (IS300.net formerly) was very focused on 1st gen IS models and has a great depth of information about that car. When the 2nd gen IS model came out Club Lexus really focused on developing resources for members purchasing this vehicle and have grown and fostered our active IS250/IS350 membership (thanks everyone).
In terms of 'approach' Club Lexus is a PG-rated work safe forum and we try to keep down the flaming and personal attacks. My.IS is a less moderated environment and in some forum areas allows nudity, etc.
Both have great info and many folks (including me) are members on both.