When different people, or “groups” practice halacha differently (I’m referring to within the legitimate boundaries of halacha), there can be negative feelings both ways. The ones who are more stringent have a nisayon to consider themselves superior to the ones who are less stringent. It’s a difficult nisayon; the reason someone chooses to be more machmir is because he feels it is better (and the poskim often use the term “Tavo Alav Brachah” on observance of a more stringent opinion). Some pass this nisayon, and some fail. Often one matures and develops as a person, and is able to improve is this crucial area of ahavas Yisroel. Hence, a common observation made is that a child who has adopted stringencies on (her)himself does not treat his parents, or others, with proper respect in this area.
On the other hand, those who keep halacha in a more lenient fashion have a nisayon to assume that anyone who keeps “stringencies’ is definitely looking down at them. An additional nisayon is to completely deny that there is anything better about following an approach which is, actually, inherently better, as discussed in the poskim (even if not obligatory).
What should someone use to determine whether or not to practice a particular halacha in a more stringent or more lenient manner? Ultimately, I hope, we can all agree, whichever way is better for one’s avodas Hashem. There are numerous variables which can affect this determination, including, but not limited to, the degree of difficulty in being more stringent (which can itself be determined by many factors) and the halachic importance of that particular stringecy/leniency. This also has many aspects to it; the stringency of the actual halacha (for example a potential aveira whose violation carries a more severe punishment), and the halachic process from which the final psak is derived. This itself includes many factors, such as the greatness and halachic status of the posek/poskim whose opinion is being discussed, and the circumstances under which the psak was issued.
It is to this last point which my comments throughout this thread, regarding chumras and kulas, have been directed.
I believe that one needs to take this into consideration when deciding whether or not to insist on only purchasing or consuming yoshon products. (S)he must also consider many other factors, as well.