Melafix and Bettafix are the same thing-- Bettafix is just a very dilute form of Melafix. They are both tea-tree oil products (aka Melaleuca). Some folks have excellent luck with these, others have had fish die shortly after using them. If you do choose to use these, be EXTREMELY careful about the dosage and DO NOT overdose.
From what I understand, the tea tree oil tends to sit on top of the water and thus when fishy comes to the surface to breathe -- BAM! He gets a labyrinth full of oil and not air.
Pimafix is not meleuca (tea tree oil) but a mixture of something else. I did notice one of the ingredients is CLOVE OIL, which is used to euthanize bettas humanely. I would therefore guess that dosage with Pimafix is even more critical than Melafix.
I personally would probably not use any of the -fix products because of the risks. I have used Quick-Cure (from Wal-Mart) successfully with a fish that was in terrible shape (though not with Ich, as far as I could tell), but that also has risks.
The general rule of thumb on here is warm, clean water, a little AQ salt, and TLC.