Don't use either, they can damage Betta's labrynth organ.
Weird because betta fix is supposed to be safe for them, but your still taking a change with it because it's watered down melafix.
I know when they are sick you can use 1 teaspoon of aquarium salt per 5 gallon for 10 days, along with warming the temperature in his home slowly. 80 degrees is great, or 85, but no warmer to help.
Do frequent water changes.
One of my bettas got white spots on his eyes (For some reason the medication I tried to use to help his fins, caused what it was supposed to treat, instead of preventing it), so I used this and I saw a very quick response with his fins and the spots on his eyes dissapeared.
Extra clean water is very important when they are ill!