One of the most common questions that I get relates to audio and the Canon 5D MKII.   My first recommendation is always to record your audio independently - i.e. with a separate device.   This gives you much greater freedom with your edit when you have a continuous sound recording - and are now free to cut between shots even if they weren’t sequential.  If you want to shoot stills and video - an independent audio recording device allows you to cut between stills and video - shot with the same camera.  

 I’ve tested the Sennheiser mics - and to be honest I’ve found the audio to be marginal - both for their hotshoe mic, as well as their shotgun mic - especially when you compare those two devices from Rode.   Sennheiser has become the de facto mic for most multimedia journalists out there - often coupled with a Marantz devices.   I’d like to suggest a new route - after some research, help from others and a few side by side tests - the Rode mics seem to excel relative to the Sennheisers - especially in the lower tones.  It’s definitely the best bang for the buck that I’ve found.  (You can ALWAYS find something better for more $$$ - this is true of everything I mention below.)