We had a very good discussion of that very topic here on the forum not too long ago. Here is a link to that thread:
Expedient Sapling Ramrod Works Fine
I would agree that the concentric growth rings of a sapling might be stronger than a billet split from a larger log, with better resistance to lateral stresses.
I think persimmon would be a good choice. It is very tough and resilient. I have a persimmon stave I cut two years ago, intending to make a ramrod, but I just haven't gotten around to shaving it down yet. It has a couple of somewhat crooked spots that will need to be straightened also. Lots of people who have never actually done it will tell you how easy that is.
Yaupon (locally pronounced YO-pahn), a species of holly, is common where I live. There are entire groves of it in places where I like to walk. It is not hard to find a straight sapling, but I can't vouch for the strength of the wood. I have a few sticks of yaupon hanging up in my shed. Southeastern natives used yaupon shoots for arrow shafts and, of course, it is also the source for the leaves used in brewing the "black drink." Arrows can take a beating, so it would probably work well enough as a ramrod, especially considering the concentric growth rings and absence of runout.
Best regards,
Notchy Bob