What's the difference between plastic toothbrushes & bamboo toothbrushes?
Toothbrushes, regardless of raw materials should be replaced frequently. However plastic toothbrushes are hard to recycle & comes in a lot of plastic packaging.
Bamboo toothbrushes are made of bamboo, that are fast growing & biodegradable. The only caveat is the bristles are made of nylon & have to be removed before the handles are composted. While not 100% plastic free, it uses much less with no packaging.
Cost-wise, both toothbrushes are comparable in price, if bamboo toothbrushes are purchased from zero waste stores.
Bamboo toothbrushes must be kept dry after use because the handles are not treated with preservatives or pesticide.Ultimately, a bamboo toothbrush may not do a whole lot for the environment in the grand scheme of things but if enough people make a small change in their daily lives, the impact can snowball & make a big difference. 
Check out Bamboo Toothbrushes here