All in one project is most optimal in vanilla, at least because there is a bug/quirk that on project completion, when you re-assign the 100 (for example) scientists on a new project or one that is below it in list, you get an instant progress as if those 100 scientists had been working for one day.
It's even possible to waterfall it : If 100 scientists are enough to complete this project, you can again put them on a third project and get again 100 man/day progress on it, which can cause it to complete immediately, etc.
OpenXcom doesn't have this behavior. A side effect is that it makes research systematically (a bit) longer.