I'm using SAI for those. An easy way to tell is that Paint.NET doesn't support pressure-sensitive brush width, so the linework ends up using one or two line weights for the whole thing because lazy. Additionally, PDN's anti-alias isn't as good, but that's a more subtle thing to pick up on.
Basically from now on I guess I'll use SAI for drawn images, and Paint.NET for manips and pictures with no Anti-Aliasing. And scribbles where I don't want to spend more than 5 minutes with a mouse.