I'd like to point out something that seems to be rather conveniently forgotten/brushed aside -
A 600mm lens is only twice as magnifying as a 300mm lens of the exact same parameters (forget the max focal length, obviously). The image gets doubled in AREA not dimensions. This is a BIG stuff to remember because a circle with double the area is only sqrt(2) = 1.4 times larger than its smaller cousin in absolute dimensions. Its not as big as you think. Worse, its MUCH smaller than you probably think.
Secondly the sensor size can compensate to a little extent to help in Zoom by cropping the image. 600mm PnS vs 400mm APS-C DSLR ? I think you'd be better off choosing the DSLR. And lenses are easily available upto 300mm for DSLRs.
OIS/Tripod. Yep. So what is that you are going to shoot with that mighty 600mm lens anyways without having a super steady hand camera ? If you need to carry a Tripod along with a PnS, then you know how serious you are about stuff.
So, if superzoom was one's priority, then they shouldnt be looking at a DSLR - you will need to change lenses and that also means that you'll need to lug around all that weight. On the other hnad if you are serious about photography, then having a 600mm lens suddenly doesnt make you special, ask Sujoy he has a massive 500mm lens - ask him the pain involved in just carrying it around.
PnS's including superzooms are all-in-one devices; stuff that you put in your bag when you go off to some place and just want a few pics to put on FB. On the other hand a DSLR is a more specialized device. Sure, it wont be able to get that eagle 200m away that your frind is checking out on his dainty superzoom, but then neither can his images be as impactful as yours assuming you are equals in skill.
I understand the need for a long lens, infact I am looking for one myself. But here on this thread I find that its being given an unnatural amount of weight. Hence the rant
No device is clearly superior than the other, no device is "the best". But if you know your needs, there is always a "best choice for your needs"