NC has two apps: calendar and appointments https://apps.nextcloud.com/apps/appointments. If your org is already using NC calendar and other apps, using appointments is great for allowing people outside your org to book you according to your preferred schedule and against your calendars. Cal.com has a few more features, but there are trade-offs. NC appointments is easily installed, integrated, and maintained within your NC install. Cal.com will require more work to install and integrate. NC is somewhat best in class rather than best of breed (The overall solution is great even though some of its apps are not as full featured). In summary, if appointments has what you need, you are probably better off sticking with it.