I've overclocked my dual-core SGS2 to 1.5GHz before I sold it but I could still switch between apps sooooo much faster using my N900. (especially with camkeyd) Also, copying/pasting text using MicroB, playing a video in the background while multi-tasking, live thumbnails during multi-tasaking etc are just some of N900's highlights which just cannot be compared with any other phone/OS imo.

I really hope the N9/50 is just as effective/efficient in multitasking.
I'll be quite annoyed if I find many things in N900 to be better than the N9/50.