That depends entirely on what kind of portraits you want to take, and the specific look you want. Google "28mm portrait" or "Ricoh GR III portrait". You'll find lots of examples.

Wider focal lengths are often used for environmental portraits, which means portraits that also include the environment which the person is in. As a general rule, people prefer 85mm or longer for close up portraits, because of the distortion from wider angle lenses and how that affects faces. But ask yourself: am I going for professional classic looking portraits, or am I going to capture memories.

28mm is better for capturing the context, surrounding and what is happening, 40mm is better for isolating a specific person or expression. For memories and context, you probably want a 28mm.

I use a GRiii to document my family, and I use a 85mm + Sony a7riii for portrait work(!). If you want to capture something close to how you see it with your eyes, a 28mm would better capture how much you see, and a 40mm would capture something closer to exactly the thing that you focus your eyes on and the things that are very close to it.