Howdy,
A treble hook gives better balance to the spinner and gives you a better shot at hooking up on a slashing type strike.(which is common using spinners) The advantages of siwash are, stronger, larger gap,less snags,can't be leveraged out. Also less chance of ripping open a finger while trying to release a fish.

I like using big spinners(#4,#5) brass,silver, pink,orange,chartruse. For Silvers try casting silver spinners upstream and reeling them fast back down to your position. Another trick is to put half a nightcrawler on the spinner's hook.

A good inexpensive spinner body is hollow core pencil-lead(roostertail's use this) painted to match the blade color. Take Care.