There are lots of motives for doing projects, but lets focus on the satisfaction of completing a project.
Complex projects may yield more satisfaction, but they take longer to complete - when does that tradeoff become untenable?
What is your comfort zone for hobby project duration?
This is a strange poll for me to formulate because I get lots of project ideas, which I write down, and periodically push forward incrementally. It could take years to push all the way to completion and sometimes they never complete. The typical scenario is I get an idea and start thinking through the solution over a couple of weeks, then start collecting components over a few months, then I finally go into a frenzy of detailed design (PCBs) for a week, then I take another couple of months to get around to going into a 2 week frenzy of building and testing. This process doesn't fit well in the poll options above.
It seems like I go through a process where I start without much commitment and gradually build commitment until the scales tip and I fully commit. Most often I won't tackle a project where I cannot complete the detailed design in a week or two, but this would often be sufficient to create a microcontroller PCB with a full set of user interface peripherals and a sensor system plus some actuators all in a 3D printed housing. I find the 3-4 month design challenges on element14 to be just doable within my comfort zone. Project 14s are a little more comfortable.
Does your project duration tolerance correlate to your attention span?