Hi G.,
I am a former nanny/current preschool teacher. I have worked with some great men in during my childcare career. As of now, my son's after-preschool care provider is a stay-at-home dad, the husband of a neighbor who has become a dear friend. My son is four and loves his caregiver to pieces. (my son also loves his Kindergarten aged daughter!)
For me, personally, I'd check everyone's references, stem to stern. Make those calls. Background check, if you would feel more comfortable. (Do Sittercity or GoNanny offer this in advance?)
Honestly, I might default toward a male caregiver instead of a mother with a baby for the following reasons:
If the baby is ill, the mother will need to stay home with it, or bring the sick child to your home. Either choice is not optimal.
If the mother is nursing, this will be an added complication/wrinkle to your day. I know this from the experience of having to nurse my son down to sleep while doing after-preschool care for two 4/5 year olds. It's not a question of the boobs, it's the job of getting the kids set up to do an activity on their own and then trying to supervise them by listening from a separate area. No real trouble, but not fun. With two siblings of differing ages, this could be more of a challenge.
Guys have their own great way with kids. I think it's important for children to have caregivers of both genders. One thing I hope will change in the future is seeing more male care providers working with younger children, because so many little ones need to have that positive influence. (There's a great book called The Fatherstyle Advantage which focuses on all the parenting assets men bring to the table.)
You can always hire your most-likely candidates to stay and play with the kids while you are home for an hour or so, within earshot of you. I actually did this with families I was interested in working for....just said "Let's schedule an afternoon for me to come and play", so that the parent could get a feel for my style, I could meet the child, and we could decide if it was a good match for everyone involved. When this is happening, work or do something else in another room, listen in and trust your gut instincts.
I hope you find the best person for your family, no matter what their situation or gender.