NZ: travelling north to south or vice versa

While planning my 5-6 weeks trip through NZ, I can't decide whether I want to travel from North to South or vice versa. Below is a list of the pros and cons, maybe someone can add some points and help me out:

North to South:
+ scenery is getting better and better - weather is getting worse and worse - more expensive car rental - more expensive ferry
+/- extra time in ChCh (slack in planning)
+/- most of the tourist do it...

South to North:
- best scenery already at the beginning + weather is getting better and better (ending sub-tropical)
+ cheaper car rental + cheaper ferry + extra time in Auckland/Bay of Islands

Don't know:
any difference in airline ticket-prices?

From this list it seems South to Nord is the best way, but all tours (and travellers I know) have gone the other way around.
What is the reason for this?

That's true, but the South is more diverse and IMHO more overwhelming.
Both parts are fantastic, don't take me wrong, it's just a matter of taste.
We want to have some slack in the planning (2-3 days), if some visits aren't possible the first time, due to the weather etc., e.g. whale watching, tongariro track, swimming with dolphins, walk on glaciers, etc. But if everything goes right the first time, we may have some extra days at the end.

For the period November - December, are there any significant changes during this period between North and South?
In other words, when you start in November in the South do you really have much worse weather as when you would end there in December?