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I was mostly asking cause the M1 I returned because of the glue issue(again, probably a 1-off situation), made me curious to whether the internals of the M1 doesn't stack up to the K&N.
They have the same design, except the outside can (K&N wins there with thicker metal and 1" nut) and filter media.
Champion Labs also have different designs for different brand/implementations, the one for Bosch/STP design is inferior, but IMHO the design for K&N/M1 is a quality one. Wix always has a quality reputation. I think it is just up to personal preference and how much one is willing to spend in many cases...