I used both and I think it really depends on what you're looking for.

Inkarnate's strengths are their assets, since they can be used to make some quick good looking maps.
I tried adding some custom assets too but I didn't find it too comfortable.

I like wonderdraft's terrain generation and the way you can add custom assets more easily.
I found it easier to use and more flexible than inkarnate, but it obviously lacks its assets.

In the end what I'm using right now is Clip Studio Paint, where I can draw directly or easily add custom assets, which I can then tag, search and drag and drop into the map.
It's also relatively easy to create custom brushes with your assets (like to place mountains, trees, etc.).
This is much more flexible than both solutions, but since it's a gimp/photoshop-like software, it's probably more suited if you want to draw a map by hand and edit assets directly (or make and use your own).