I know a lot of people like Bu Jin Design's bokken, but I find them a bit too club-like for my small hands, even the "small" size.
The bokken I most often use are two I bought from Kiyota Company in Baltimore, MD. One is what he called the "Hombu" style and doesn't have much of a sori. The other, I believe, is a really light-weight Yagyu Shinkage-ryu style bokken which is nice for "quick" work and also for two-sword practice.
The best thing people can do in choosing weapons is to hold them in their hands before buying them.
To bring things back on-topic, I actually do think that the material of the weapon matters. If you're going to be doing kumitachi and kumijo, it's inevitable that your weapons will get hit. I've seen cheap bokken splinter easily before which may lead to injuries.