- According to Jewish philosophy which is more important in Judaism, keeping Shabbos, or treating others decently?
- According to Jewish practical law, which is more important?
None of the בין אדם לחבירו overrides Shabbos. There are a couple of reasons:
Mitzvot are ordered hierarchically according to their punishments. There's no way of transgressing Shabbos with the capital/corporal punishment for a באל"ח Mitzvah that is either ממונות with no punishment but repay or general Mitzvah of Chesed that can be forgiven (the reason we don't say blessings on those Mitzvos).
The Gemmorah in Kiddushin 30 is clear about preferring Shabbos over כיבוד אב based on the Pasuk "איש אביו ואמו תיראו ושבתותי תשמרו אני ה'". The reasoning is simple - since both of you have to respect Shabbos there's no way of performing a באל"ח by transgressing Shabbos. Please note that כיבוד אב is thought to be a very important Mitzvah comparable with honoring Hashem.
Shabbos is called ברית ("את בריתי תשמורו"). Keeping Shabbos is a way of testifying that Hashem created the world etc... None of the באל"ח Mitzvos is called ברית.
Shabbos has both Do and Don't parts, where most of the באל"ח are only Dos and we have the rule (Rosh Hashana 32) Do's don't override Dos with Don'ts. THere's a long dispute there to understand why Shabbos is so serious.
Clarification: as with כיבוד אב, there's no clear-cut between those two types as ALL of the באל"ח include באל"מ, also all life-threatening Mitzvos (murder or life-saving) are more באל"מ as a man is created to resemble Hashem's. So those two do override Shabbos.
That's really a tough question. On the one hand a person is defined in their religiosity by whether they keep shabbos or not but on the other hand both are mitzvos and pirke avos tells us not to distinguish between mitzvos. I don't think either are more important than one another but they address different sides of the spectrum of what it means to be a religious Jew