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.
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