Canon 5Mark III vs M series
If weight is important you should go for the EOS M. But even though you will be able to use your current lenses with an adapter the M camera has a crop factor of 1.6. And you will not have the comfort of an optical viewfinder. AF should also be slower with the adapter and EF lenses.
Anyway for sure I will take a look at the new EOS M when it hit the stores.