IMHO WavePad Sound Editor is superior. The sound quality is maintained even when you slow it down to ridiculously slow, like 10%. Try downloading the free trial version and see if it works for you -
http://www.nch.com.au/wavepad/index.html
I haven't used riff master so can't comment on it.
Cheers
[FONT=Franklin Gothic Medium]Youtube Channel - Jim47ww[/FONT]