Personally I prefer straight ones, they are easier cut to size if they need to be shortened and provide the same profile for the whole length so remain the strongest.

A rattan bo makes a cheap bo for first time users and is quite good because you don't get as many bruises as you do with a hardwood when you are learning to move it correctly and with force around the body. The shock factor on the hands is less too.

I have several rattan ones ranging in diameter up to a really thick & heavy one I use for kumibo. I have several oak ones but the heavy rattan one is my favourite.

For jo you need to get a very dense hardwood either red or white oak is good, look for small densely packed grain and it should last reasonably well.

I have an octagonal bo. It would penetrate a little harder, would be less likely to have a strike hitting a flat surface slip off, and best of all never, ever binds up in my hand.

Where do you get one of thoese?

My experience with octagonal bo has been that they seem prone to warping and they break easier. I think if you look closely at the major okinawan kobudo systems you will see that the only octagonal weapons in use are short ones.