Well, I don't know about the first part of your question, but as for the MyWi, it depends on your configuration.
For instance, I turn the pulsing blue bar off and drop the strength to about 30% and I find the battery drains about 12% an hour.
Usually, I connect my phone to my MacBook so that it stays charged while I'm tethering. It doesn't drain the MacBook battery as much as running it alone drains the phone's battery.