If I connect my iPhone to my Mac via USB, it enables personal hotspot on the phone and then my Mac sucks my data-plan, even if I'm connected to WiFi on both devices. I don't know why, is there some kind of setting I could disable this behavior? I want to prioritize WiFi over celular data, which are paid and limited.
If your personal hotspot is on all the time (even when you are not using it), your iPhone will tether its 3G connection to any Mac or PC with iTunes by default when connected with a usb, and you can do nothing to change this behavior. You have to turn it on only when you’re actually using it, and you have to keep it off if you do not need it.