Let's say you are interested in 2 startup ideas, both roughly equivalent in opportunity size and personal fit, but in different markets. There isn't a factor that that makes either stand out. How would you decide which to pursue and break analysis paralysis?
Build a simple landing page for each idea. Put a sign-up button on each page. Have it open a form that accepts email addresses as a way to register interest/receive updates. See which idea is more popular.
I'd make a video about how each one works, put it on their own landing page and market it like crazy. Email / signup is too high a commitment to ask from potential customers but people seem more receptive about giving anonymous feedback