Where is the better ROI ? The question is not easy to answer, because you have to account not just for the money you invest, but also for the time spent.
A developed site could make more money than a parked domain, but is the time worth it ?
Fresh contents usually have to be paid for, unless you can write your own. It takes time = money.
If you set up a real business on a site (for example dropshipping), it doesn't run on autopilot all the time. Promotion takes time. Even setting up free & open-source software takes time, assuming you have enough technical skills to run a website and configure your webserver.
Finding domains takes time too. Scrubbing through expired lists takes time, contacting domain owners and negotiating takes time. Handling and renewing inventory takes time. Other than that, you can be passively holding domains and wait for offers. Domain names do not require constant maintenance, while websites need to be promoted, maintained and monitored.
Domainers should act like professionals, keep timesheets to track their activity and put a dollar value on each and every hour devoted to their trade. This in addition to proper accounting (revenue + expenses). Then figure out for yourself where your time has been most profitable.
Developing websites and speculating on domain names are two different activities. Not everybody will excel at both. It's not like you can easily switch from one to the other imo.