[duodecad]

Well, this is probably not what you want to hear, but my answer is "both". I'd say my projects are split about 50/50 between parallel and sequential.

Maybe this can be a preference that the user sets individually for each project, when they create it?

I suppose if I was really forced to choose, I'd go for "sequential"-- my logic here is that a parallel project can be set up and worked on as a sequential one (by simply cranking through one task at a time), but a sequential project can't really be done as a parallel one at all-- the dependencies wouldn't allow it. So if I really had to choose only one, I'd go with sequential.