Greenhouses, Sheds and Sun Rooms - Running water line to greenhouse?

I've had a greenhouse for a few years now, and have supplied water to it in the warmer months by running a hose from a hose bib/multi gang connector into the greenhouse itself. I'd like to run a real water line from the house to the greenhouse, attaching below the kitchen sink and then running it through the garage. The greenhouse is a lean-to that is attached to the garage.

The only problem is that the garage isn't heated, and we get freezing temperatures in the winter. I think I have 2 options:

1) Run the water line, and then wrap a heat tape around it for its full length and insulate around the pipe, or

2) Create a low spot with a valve and drain the whole line every winter.

I'd rather keep the line active if I could, because I still do some watering in the winter. One question I have is whether I should use 1/2" copper or 1/2" PVC? Is one less likely to freeze than the other, or does it make no difference? would there be any benefit to going to 3/4 instead of 1/2"?

Thanks!

Doesn't make a lot of difference what kind or size of pipe you use -- if it gets cold enough to freeze, it will.

It would be more work, but if you can install the pipe inside the wall common with the house for as far as possible, that won't freeze. (Use copper, not PVC, if you do that!) Insulate the rest of it, and install the drain. Watch the temp., and drain the pipe when it's expected to drop below some point -- like 15 degrees or whatever.

Or heat tape. That'll work too.