Dave has a good description of the difference between blogging and podcasting. There is a huge difference between the written word and spoken word. Neither is neccessarilly better or worse, just different. The same words can be given a very different meaning by the way they are spoken.
Every medium has its strengths and weaknesses. The great thing about podcasting is that you can take it with you, driving, running, walking, where ever, and just let your mind absorb ideas through your ears, which is a very nice way to get to know someone, very different from reading what they write. Where the written word only has the intonation the reader can infer (adding a lot of errors in doing so) and can be skimmed (lowers comprehension by orders of magnitude) the spoken word has great subtlety and subtext, and can seduce, cajole, wink, leer, lurk, and be lewd. Both forms of expression have their place