In theory, there is a bandwidth advantage to a larger diameter, but at 20 meters, I doubt you could detect the difference between copper tubing and #30 wire.

My advice would be to get what you can get easily and cheaply. If there isn't much traffic in the attic, I'd probably go for #18 or 20 wire, I would prefer stranded because of it's durability, but if it's protected, that's not a big issue.

You may find larger wire easier to come by at the local Home Depot or whatever, and larger isn't much of an issue except it's harder to work with. In particular house wire has an annoying insulation that doesn't strip as easily as hookup wire, and you will find that soldering iron you use on SMD devices a little puny with #12 house wire!

On the other hand, house wire is intended to be run through the walls and from a fire safety perspective will be better than hookup wire, but since it will probably be in the open and horizontal, I don't think that will be much of a factor, either.