disclaimer: i haven't used proboards in a long time so my information on it may be dated / incorrect. also, i have a clear preference.
now, i might miss a few things but here are the pros and cons i have:
proboardspros- beginner friendly control panel and basic customization for forum creators.
- has a library to easily find and upload plugins/scripts and themes for anyone to use.
- custom domain support, albeit for a monthly fee.
cons- some of its features are restricted by pay requirements such as the amount of plugins.
- does not offer the ability to include mature content, even with payment.
- html support in posts requires a plugin and learning a new language based in bbcode to implement your templates.
jcinkpros- all of its features are free to use and customize.
- html is supported on most, if not all pages.
- has a lot of quality of life features baked in such as sub-account switching through a dropdown.
- offers mature content, albeit for a monthly fee.
cons- plain control panel and non-previewed customization which (based on feedback i have seen people say) can be intimidating for forum creators.
- difficult to find scripts or themes if one doesn't know where to look. the access to this information is scattered and even gated.
- custom domains are fickle and have little support. john doesn't seem to be in any rush to get these working at 100% either.
- some scripts / features that proboards has are just not available.