See
Mobipocket Decoder Tool and
eReader Decoder Tool. I'm not sure if anyone has done a direct test of MOBI vs eReader as the source for format shifting, but my impression is that either is ok.
Based on DRM free files, there is little doubt that sometimes MOBI is worse than LIT for format shifting - but it is usually "good enough". There are relatively few DRM-free eReader ebooks, so this format is harder to compare with LIT and MOBI for format shifting.