unconventional vs. nonconventional (or non-conventional?)

Nonconventional is a rarer alternative only in a few dictionaries, but with essentially the same meaning.

Spelling: Merriam-Webster has "nonconventional" without a hyphen so that's evidence you don't need it for American English. However, Lexico (Oxford), the online OED, and Macmillan don't have the word. Spelling it "non-conventional", i.e. with a hyphen, might be safer, especially in the UK: hyphenated it's not wrong and may be easier to read for people not familiar with the word. There are several Wikipedia article titles starting "non-conventional", for instance. "Unconventional" is happily unhyphenated, in OED, Lexico, Merriam-Webster, etc.

From the above, we can also deduce that "unconventional" is more common and more accepted.

Definitions in Merriam-Webster of both are very similar. Nonconventional is "not conventional : not conforming to convention, custom, tradition, or usual practice : unconventional" while unconventional is "not conventional : not bound by or in accordance with convention : being out of the ordinary". No real difference.

Is there any difference in usage? Merriam-Webster has for "nonconventional": "nonconventional teaching methods", "nonconventional energy sources" and a quote about a "nonconventional source" of ideas. I can find many examples where all of these are replaced by "unconventional". In general it seems "nonconventional" is used in the same way as "unconventional".

One difference might be that "unconventional" is often used of lifestyles, clothes, behaviours, etc, (e.g. Forbes, India Times) with the sense that you're deliberately going against the normal or are seeking out an alternative or non-mainstream way of living: this might be criticism, or it might be a positive thing if you're the sort of person who sees yourself as a rebel. Phrases such as "unconventional lifestyle", "living unconventionally", etc, crop up a lot. I get the sense that "non-conventional" is considered less of a value judgment than "unconventional", as in the Wikipedia titles where the goal is neutral point of view. "Non-conventional lifestyle" seems much more neutral both in my impression and in a lot of scientific-sounding Google hits.

Despite this, the summary is that nonconventional/non-conventional is a rarer alternative with essentially the same meaning. Sometimes people don't think a word sounds right or think it has some connotation they don't like. There are plenty of words starting "noncon" or "noncom" ("nonconformist", "noncommittal", etc), as well as lots "uncon" or "uncom", so it's logical to use either "non" or "un". The one difference is that "unconventional" when used of lifestyle carries hints of the countercultural or rebellious, while "non-conventional" sounds slightly more scientific, but it isn't a clear division.