[Draug Isilme]

If your betta seems to be happier in a 5g, definitely do away with the 1/2 gallon! Well, personally I think you should keep it on hand for possible quarantining needs in the future >.> or when you're changing the water.. but other than that I'd say the 5g should be his personal home! ^.^
1g is usually the advised minimum, but main reason why people are against even the 1g tanks is because it doesn't follow the 'inch per gallon' rule, it's actually more maintenance to keep good water quality in comparison to a larger tank, and they're near impossible to hold a cycle.

 

[Draug Isilme]

I'm not really a fan of the inch per gallon rule either.. It's more like a guideline to me since when you deal with fish like goldfish, you need 20g+ and common goldfish grow to 12 inches and other fancy goldfish are more or less 6-8 inches.. and then you have snails which do vary on the size, but again most times they apparently have a huge bioload but shrimp can grow to an inch but have a TEEENY TINY bioload so I suppose unless specified the inch per gallon rule is just something to go by unless certain requirements (like bioload or heating, I'm suprised at myself for forgetting to mention that >.<) then it's better to follow it...