I’ve used Invision for my community before migrating to Discourse (doesn’t anyone else get bothered by the typo in the title 
).
I guess the choice depends on how big & mature your community is and your specific use case. Yes, groups in IP were great, but in my case they weren’t utilized well enough. Partly because forum users rarely self-organize, partly because there wasn’t a lot of need to it.
I and my users found Discourse to be lighter (both in terms of UI and UX), easier to use, and less “crowded”. My community has been growing faster since we made the switch.