Update: I recalculated the things and noted that if I enabled first Haiku, members used it as a new thing. They generated in a few days a few hundred summarization ~800. Than we can go higher because those topics has been summarized already and these will reduce the usage. So now we are using Opus after a few days Sonnet usage and Opus is a whole new dimension. 
Opus can recognize images and can makes statistics about the topic posts. I mean e.g. there 5000 posts in topic and Opus can categorized the replies and list the most replied things and makes suggestions based from posts. I think Opus is the best I’ve ever tried, yet. 
So in my opinion if you start with the cheapest which is now Haiku then you can go upper to Sonnet and go on because the usage will reduced. Which is cool because you have the option to check every model. 
But it’s really depends on the forum usage of course…