Allright, first of all, I said it was the best
in my opinion, I didn't say it was the best
in everyones opinion.
Second, like I earlier said, I didn't know you already knew IPB. I gave an URL so you could check it out and maybe consider it as being 'better'.
Third, read both the features of Invision:
http://www.invisionboard.com/switch/ and Discus Pro:
http://www.discusware.com/discus4/pro.php and keep in mind that Invision is totally free and Discus Pro costs 'only' $149.95
I could go on and discuss some of the features and how they could come in handy for this site, but I'm fine with this forum also, it does it's job very good, all the other functions are just 'luxury'. Seeing you already purchased discus pro, have members with posts, profiles and uploads, I can fully sympathize with your descision to stay with it, since there is no convertor for Discus Pro to IPB.
And for the last "back to this forum issue.." sentences; I never said you were wrong. And even though I never said that IPB is superior to Discus Pro, I do believe that's the case, since it's free, has far more options and simply looks better.
I also never heard anything about Invision being hard to manage after 2 years of use with 10.000s of messages. But extremely busy sites seem to be doing just fine with Invision.
Anyway, so far for my backup of argumentation. Feel free to ask some more, even though I thought Invisions site itself would be explanatory enough.