What to look for when purchasing a headset
I've had problems in the past with headphones feeling cramped on my
  ears, which I don't want to be the case with my headset


Do your best to test them out before you buy. Some pilot shops will let you put them on your head and if you are nice to the people that lurk around the airport they may lend you one for a flight to try out. I find that the Zulu's have bigger ear cups than the Bose but I don't have any hard numbers on size this is personal experience wearing them. 


  First of all, is noice-cancelling actually that important(in terms of
  health and comfort)


Yes, just ask any one that flew in the pre-headset days. I personally find that I can comfortably fly for longer times with an ANR headset. Another thing to note is that the ANR head sets tend to not squeeze your head quite like the passive headsets I find this to be the real gain in comfort. 

One of the big functional differences is that some of the LightSpeed headsets do not cary a TSO, I belive all of the Bose do. This may impact your ability to use them for commercial purposes in the future. You can find a discussion on it here.