I think most people that have fitted oxygen tend to use that, but sometimes they can't because if say a plane is moved from N to G the CAA certification can be prohibitive so they use a portable kit.
Re masks, they also use a lot more o2 than a cannula - of the order of 2x to 5x more. That is a serious problem, but luckily there is rarely a need in N Euro weather to be above FL200 for more than say an hour at a time.