Libsyn hosts by the amount of space you use on their website, but has unlimited bandwidth. From what I have read, they are also the most reliable.
So, I chose the lowest payment plan, which leaves me three episodes of 20 minutes a month, or two episodes of anything longer.
I think you misunderstand how LibSyn works (I have been hosting with them for 2+ years now). The lowest account of 50MB per month is on the "fast" servers. Anything over that is shifted to the "slower" servers, as they are considered old episodes and not as desired/downloaded as often. From personal experience, this is true. I get the most downloads in the first 2 weeks a new episode is posted and then they trickle after that.
The files will always be there. If you go over in a single month, your oldest file(s) will be shifted to the slower servers automatically. They don't cut you off and the files will always be available.
Also, if you compress your audio properly, you will be hard pressed to fill 50MB a month with a weekly 20-minute show. For example, my latest show was 9:42 and was only 5.6MB in size. My past 4 shows totaled 56 minutes and were only 31MB combined. So with 4x20 minutes a week, you are looking about a little over 40MB in space.
Pure spoken word does not need to be big on file size. I publish my audio at 80kbps in mono and people always remark on my high audio quality. Stereo needlessly doubles the filesize for spoken word.
When you export your files to MP3, use the following settings:
Sample Rate: 44100 Hz (44.1 kHz)
Bit Rate: 80 kbps
Your biggest source of improvement in your audio quality is your microphone. A good microphone makes ALL the difference (and one that works well with your voice).
LibSyn rocks as far as service and stability. My only issue with them are their stats reporting. Stats are inconsistent, often down, often delayed and questionable at times. I always download the source log files and run them through my own analyzers to get the real numbers, and they can differ by several hundred with what is reported on the site.
Good luck with your show!