There are 2 cases, DRm'd or not.
When DRM'd :
If a Mobipocket file is available, I buy it (the best formatting generally).
If not available, I buy the LIT version, then use ConvertLit to generate an OPF file, then Mobipocket Creator to build a PRC. If the formatting of the PRC is not so great, I try with Calibre.
When not DRM'd, Multiformat then, I try first the PRC or Mobi file, and if the formatting is not great, I try the LIT file then ConvertLit and MC.