Sink. I think shower brushing is gross. Don't ask me why because I can't tell you why; it's just gross to me.
I also brush about 87 times a day and have toothbrushes everywhere, including at work (of course), in my purse, and in the glove box of my car. I have a tiny toothpaste tube in my purse too. You just never know when the urge or need to brush is going to happen.
If you're brushing your teeth two minutes at a time, BE CAREFUL. That's a good way to wear your gums thin.