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!