It'll be interesting to see what happens, will the 2 sites continue much as they are now, or will they become one single entity? I personally use WIMA most of the time, only resorting to IMSLP when I can't find what I'm looking for, hence I know my way around WIMA, but find IMSLP more difficult to navigate, I suppose that would change as one gets more familiar with it.
What I mainly like about WIMA is the inclusion of MIDI or MP3 sound files albeit electronic synthesized and often Finalé, Sibelius, MusicXML and other formats. If these are lost in the merger then I would be most unhappy. I've not yet found any similar facilities on IMSLP. I use Finalé Songwriter myself and I can usually find a work in a format I can import from WIMA, a converter for PDF to MusicXML costs $199 (assuming it works) which is not justifiable on a pension and why I prefer WIMA.