AFAIK, the FA 100-300 is a significantly better lens than the FA J, both optically and in build quality. My understanding is that the FA J series was a very low line -- a beginner class below what is commonly called "consumer" class. I've see a number of samples from the FA 100-300 and they are okay, especially stopped down, but it's not a great lens, especially if you want to shoot it wide open. I could be wrong, but I believe the FA J is built with a plastic mount and has no aperture ring.
Of the Tamrons, the 70-300 LD Di is considered a much better lens than the 75-300 (and it costs more), and is probably in about the same class optically as the Sigma DG APO Super. Pentax plans on releasing a 70-300 in about the same aperture range later this year (?), but will probably be more expensive.