There was a suggestion from HP Ron Orenstein that auks are more diverse than penguins. Technically this is true. However, the inference that is drawn from this--viz., that the penguin niche is just as fruitful in the north as the south, and it's only because of competition and/or lack of transport in cold currents that they don't invade the north--does not follow.
This was not an inference I intended, though I suspect that the northern seas would be better habitat for penguins than tropical waters. Remember that auks and penguins are almost unique among seabird famiilies in lacking trans-equatorial migrants - a factor probably resulting from the evolution on migratory patterns in auks and, of course, flightlessness in penguins. Such species as Arctic Terns, Slender-billed Shearwaters, Wilson's Storm Petrels and many others breed in the high latitudes of one hemisphere and winter in the other. Though this does not suggest that the two areas are equivalent - after all, the winter grounds of many land birds differ greatly from their breeding grounds, and the birds may show corresponding shifts in diet - but at least it shows that the same species can exist in both the arctic and antarctic.