Attention to the breath: sensations at the nostrils vs sensations at the abdomen, as they relate to serenity and insight During sitting meditation, I attempt to anchor attention to the breathe. I have read about anchoring on the sensation(s) of the breathe at the nostril, and I have read about anchoring on the rise and fall of the abdomen. Both techniques appear to be in concert with Theravada approaches. I recently read (Wikipedia article re Mahasi Sayadaw) that Mahasi Sayadaw taught attention at the abdomen. I need an explanation, please, of the two different approaches ... the impact of each on serenity and on insight. Perhaps one is "better" for serenity, and perhaps one is "better" for insight. Thx!
It really doesn't matter which location you meditate on. The only big difference between the two is that the movement of the abdomen is a very apparent thing while the movement near the nostrils is much more subtle and harder to catch.

The thing that determines if you are practicing Vipassana-Bhavana versus Samatha-Bhavana isn't the location of what you are meditating on but rather the way in which you are looking at the object of meditation. This is clear if one looks carefully at the instructions in Chapter 18 of the Visuddhimagga. Paragraph 5 of said chapter describes Vipassana according to the four elements saying:


  
  But one whose vehicle is pure insight, or that same aforesaid one whose vehicle  is serenity, discerns the four elements in brief or in
  detail in one of the various ways  given in the chapter on the
  definition of the four elements (XI.27ff.). Then, when the  elements 
  have  become  clear  in  their  correct  essential 
  characteristics...there become plain ten instances of materiality 
  (rupani) with the body decad...
  


Later in the chapter it is described also in terms of the 18 Dhatus, the 12 Ayatanas, the 5 aggregates, and again the four elements. All of these different categories are classified as being ultimate realities within the Theravada school, whereas everything else is classified as Paññatti, or concepts, meaning that they are mental representations of experiences rather than the experiences themselves.

Thus the only thing that really makes a practice Vipassana meditation is that one is meditating on the object as an experience rather than as a concept. If one is meditating on the abdomen and sets their mind on the physical feelings of the movement themselves, then one is practicing Vipassana meditation but if they are meditating on the stomach itself as being an object, then that is not Vipassana meditation.

Similarly if one is meditating at the nostrils and one is meditating on just the feelings of the air that happen at the walls of the nostril or the lip, that can be Vipassana, but if one is meditating on it just as air moving in and out, that is not Vipassana