This blog post should be obvious by itself, but I think I will add some more things to help you decide on which file format is the most suitable for your podcast.
The short answer: MP3.
MP3 file format is the de facto standard in audio podcasting. It is consumable on most, if not all, modern PC and Mac platform. This is also the format which portable media players are known to be compatible with.
After all, why they call it MP3 player if it can’t play MP3?
If you know your audience will be mostly on a Mac platform, or use iPod or similar portable media devices, you can alternatively choose AAC.
AAC is a modern audio codec that is part of MPEG-4 specifications. It deliver higher quality audio than MP3 codec at any comparable bit-rate.
WMA is the format compatible with Windows platform. MP3 typically produces higher quality sound than WMA unless in low quality setting.
Finally, there is really nothing wrong in producing the podcast in both formats. The thing is, you also need to generate two different kinds of podcast RSS feeds. RSS specification allows no or one enclosure per item, so unfortunately, it is the only way to handle that.
Although there are RSS and podcast aggregators that support more than one enclosures per item, it seems like now having multiple podcast feeds is the best practice.
I have noticed podcasters provide links to the secondary file format without feed. In this case, for minority users out there, while you can get your favorite file format without manually converting it, you will have to download it manually.
Also read: Podcast tutorial.