I had a bad experience with self adhesive ones but that could had been a cheap one.

+1 on the spare tube. However, if the other one is sound, you are better off fixing it. Some tubes have micro holes. Those are the ones that you need to put air once every week or two. Every once in a while you find a tube that holds air for months, those are golden and superior to an unproven brand-new tube. My front tube is several years old and have four patches on it but it never loses air.

I repeat, tire boot, or a piece of canvas. If you put a gash on the tire itself, you won't be able to patch the tube and go. The dollar trick will do (fold a dollar, put it inside covering the hole, dollar material is strong enough to stop the tube from gushing out) but if you are already messing with the tire you might as well fix it right from the get go.