I have both, and bought IM+ originally because it was the only workable multi-protocol IM client for the iPad. I was (and am) using Beejive on the iPhone.
I don't care for the UI on IM+ at all. It may have more features, but it just feels "off" to me (I know, very subjective). So much so that after a day of using it on the iPad I went back to using the iPhone version of Beejive in x2 mode until the native release popped up.
The browser is the only unique feature of IM+ I do like, but I'm still unsure why they put a floating semi-transparent button in the message text area to switch to it when they have most of their tool bar sitting empty.
I also had issues with notifications in IM+. To a certain extent that is mitigated by it having an in-app browser (you CAN open links in Beejive without leaving the application, but you don't have an address bar), but if I was using it for IM and NOT browsing and instead using another app then it WAS an issue. Could have been a temporary server issue, just a shame it clashed with my usage.
I might let IM+ hang out on my iPad for a bit and see where it goes, but I can't see using it seriously at this point. But each to their own
I DID apprecaite that IM+ was a universal binary, even if there is no way in hell I'd ever use it on the iPhone.