I'm hardly an expert, but I think it depends on what you want and how you want to use it. I know mid-drives are great for folks who climbs lots of hills. Hub motors aren't as good on hills but are great on flatter terrain. Both are fine for someone who wants to run the motor all the time on their rides. I've never had a mid-drive but I had a 350w front hub bike at one time, and it suffered on really big hills (Bay Area) and got super hot. Friction drives are, I think, for someone who wants to just have some assist maybe on hills and in winds or to keep up with others on rides, but wants to preserve the human powered nature of their bike...especially if they have a lightweight bike. What do you want? Or are you just looking to build what seems interesting?