Is it easier or better to use a same sex bathing assistant with the senior or does it vary from senior to senior? Do senior males prefer men?

I don't think men care as much as women. My Mom had Dementia a fought the male aide they sent into her room. I don't know why, but she disliked men when she contracted Dementia. I had to ask for a female aide. Me, I want a female, no males.

My dad didn’t care either way, but I’d definitely say it varies person to person

It will most definitely vary from person to person.
When my late husband was under hospice care in our home, the hospice aides that came to bathe him were females and he was fine with that.
However when hospice sent a male aide to bathe him one time, my husband was not happy about that at all and I had to call hospice to tell them not to send a male aide again in the future, which of course they honored.
So just ask which sex the person would prefer and then try best as you can to honor their wishes.

Ask your senior if he has special wishes in this regard if you have choice in the matter.