I guess I prefer medium length common routes? For longer common routes, it reminds me of Code:Realize's common route, which I thought was waaaaay too long, it took like forever for me to skip through it with the skip function just to play other routes lol. But I also want the common route to be a little longer, so you can interact with everyone before being put into their routes. I much prefer medium to longer character routes, though. With short ones, either it seems like the romance happened too quickly/came out of nowhere, or there's barely romance at all. But at the same time, if a character route is too long, and isn't interesting enough to me, it becomes a slog.
But if I had to choose out of the options, I would say short common route, and long character route.