>>2010358Try and keep a more constant thickness throughout your lineart. As it is, it has too much variation within the same lines. If you aren't doing so already when tracing the lineart, try to lay down your nodes in pairs and keep their handles in parallel. This helps avoiding lumps.
Overall, lineart is a bit too thick in general and way too irregular in it's thickness. You'll get better at this with practice. Also, I haven't seen the original but I feel like some of those ends could've been sharp points instead of being rounded. I've circled a few examples.
And for your friend, it comes down to how much effort he's willing to put in his vectors. Strokes are a tool at your disposition while fills is a technique. With fills, you have freedom over the thickness overall, variation of the thickness, how to end you points, giving perspective, increasing focus on certain parts by making the lineart bolder and it also avoids a really flat and mechanical looking lineart when that's not what you are looking for.
