Which do you use (assuming you use one of these two)? What are the pros and cons?

It appears that such teachers Mahasi Sayadaw and Ken Folk suggest the abdomen, while such teachers as Pa Auk Sayadaw suggest the nostrils.

If you have made serious progress in 1) insight, 2) concentration, or 3) both, I am particularly interested in your thoughts as that would kind of prove that your method has produced awesome fruit.

I have been focusing on the breath at the nostrils and doing samatha meditation (I have as yet to achieve jhana or piti), and am debating 1) switching to vipassana noting practice, and a) continuing to breath at the nostrils, or b) changing the focus to the abdoment, OR 2) staying with the samatha meditation and a) keeping the focus at the nostrils, or b) changing the focus to the abdomen.

Thanks!

It might simply be a personal matter.

I personally alternate between full-body focus, focusing on the surface of the skin, and focusing on the lower abdomen (4-6 fingers width bellow the belly button).

I have always failed to have lasting pleasure while focusing on a single spot one-pointedly, and in general I always try to keep it mostly wide — so even when I say that I "focus on the lower abdomen", it's more like I have a very wide and diffuse focus, with some preference or inclination towards that region; Even Then, I try focus on the "whole" of the lower abdomen, avoiding to fabricate an imaginary "single point" that reifies "the lower abdomen".

But I have known a practitioner who did the exact opposite: he focused on the passage of air along the skin above the lip and below the nostrils, while construing a "single-point" which gets smaller and smaller the more concentrated he was. He told me that eventually it isn't even the skin or the breath anymore, just this (fabricated) "center of focus". And this person was very successful at generating pleasure using this technique.

I.e., it is reasonable to try several things before deciding to move on to a different practice, and even if you do move on it will be worthwhile to go back to the original practice and try it again after gaining some expertise in whichever new practice you take on. I think that this is the general approach taken by many practitioners.

I say go for whichever you are best with. Initial successes build confidence and objects that are easier to perceive make for more initial successes.

All objects are valid objects for insight practice, as all demonstrate the 3 Characteristics, so there nothing magical about one vs the other.

Either can be used just as well for insight or more smooth jhana, so either will be fine.

Daniel