I have the Bose, I bought my mum the Sennheiser
The Sennheisers work without a battery, the Bose do not.
The music quality is better through the Sennheisers, the noise cancellation is better on the Bose, which means in noisy environments you hear the music better.
The Bose are a lot more comfy
If buying a new pair I would buy the Bose again and not the Sennheiser. The Bose sound cancellation is the best I have heard
The best bet is just to go and try some out, one persons headphones may sound great to them but crap to an others.