Despite AFF's popularity, it is no longer being actively developed by Simson Garfinkle, its original creator. In fact, the current version of Guymager no longer has AFF disk image support enabled by default. From the Guymager wiki, "Simson Garfinkel, the inventor of the AFF format, recommends not to use his AFF format any longer. He sees no need in continuing with AFF and maintaining related libs and tools since EWF became so nicely documented thanks to Joachim Metz."
To answer your question, then, I would recommend using Expert Witness Format (.E01) forensic disk image formats for digital preservation.