I was lucky... the J&J was what they had in stock on my vaccination day. But, at the time, everyone was convinced by the published figures that the mRNA vaccines were better. In fact, I got my shot date moved up because someone walked out of the clinic, as they wanted the mRNA vaccine instead.
I heard one doctor suggest that women (particularly younger women) are better off with mRNA and men with J&J. Children with only one dose of mRNA. (If at all.)