These are just ideas and things to experiment with.  But, if you choose the nostrils and really want to focus on the breath, you really have to be careful not to get too tight.  So, where you might notice tightness is in the eyes, in the hands, and in the end of the outbreath.  So, if you can keep the end of the outbreath really relaxed and just stay with it, the hands might have an enlarged feeling and the eyes will be relaxed.  That can work well.