Would a trait that has only two distinct phenotypes more likely to be a single-gene trait or a polygenic trait?

single-gene

          ...         A       ...     a
A              AA             Aa
a               Aa              aa

It would most likely be single-gene.  For each gene that is involved, more phenotypic options usually become available.  For example, if a trait were caused by two genes, each one having just two alleles, there would be four combinations of alleles, which could (but wouldn't necessarily have to) result in four distinct phenotypes.  Most traits that have a variety of phenotypes, such as hair color, eye color, and skin tone, are polygenic; each gene contributes a small part of the ultimate phenotype's characteristics.  But a single-gene trait with two alleles is just one phenotype or the other; it's like an on-off switch, black and white with no grey in between.

A single - gene trait.

Polygenic trait,such as height in humans, would show a wide range of phenotypes.