Wow! This is an old thread, but I'll respond anyway, just in case it's of use to anyone. In my opinion, it's probably better (if you're not sure) to begin with a lighted weight bokken as a beginner. As you progress, you will get stronger, hopefully develop good form, and then you can move on to using a heavier weight bokken. Also, I think that when you consider the strength or softness of the wood, consider what type of bokken your partner is using. The most important thing is that you have bokkens of (approximately) matching strength or weight for practice. This will help avoid breaking your practice tools!
Personally, I like to use white oak weapons, but there are many times that I choose something else because of circumstance (the type of weapon my partner is using, how strong/good my muscles are feeling that day, etc...) Please, look at our website
http://www.homestead.com/floatingmountain/ where we sell hand crafted weapons from a variety of woods. If you have questions about woods, sizes or weights of weapons, just e-mail them in. Thanks and good luck!