In Finland unlimited data plans are the norm, so using online providers doesn't bring any cost. And online providers tend to be much better quality in terms of map rendering, search query parsing, navigations instructions, etc.

When travelling in Europe, I tend to just download Google map tiles and copy them to the device. After that, only searches and navigation cause data traffic, and those are just small JSON files, it's such little traffic that it's not a problem in Europe these days.

When travelling outside Europe, offline maps could be useful, but that's rare for me.