I can't speak much to Inkarnate, though from what I have seen it is very limited stylistically. If you like the Inkarnate art style, then perhaps it is worth paying for, but otherwise it seems quite limited compared to Wonderdraft.

I am primarily a Gimp user, but I started out using Wonderdraft quite a bit and I find it to be very versatile in what you can do with it. I still use Wonderdraft in my workflow for some maps, and then export as layers (Multiplane) to integrate in Gimp. The nice thing about Wonderdraft is that it is highly customizable and allows you to create in virtually any style you can think of.