18V or 20V battery for my cordless tools?

The question is kind of a trick one because in practice you'll rarely find new 18V tools anymore - manufacturers switched massively over to 20V (or higher) a few years back. If you're choosing between what's actually available, 20V gives you more power and runtime for pretty much the same price, especially if you're planning to keep your gear for a while. Just make sure you stick with the same brand so you can swap batteries between tools, otherwise it's a pain.