AFAIK TSdfDataSet is a (Comma/tab/?) seperated file and TFixedFormatDataSet is a fixed width file where all the records are the same length whether theres data or not.
SDF Sample : Name,Surname,Tell,e_mail
Rec = "Piet,Pompies,,piet@pompies.net"
Fixed Sample : Name = 15 chars; Surname = 15 chars; Tell = 10 chars; e_mail = 20 chars;
Rec = "Piet Pompies piet@pompies.net "
Hope it helps