Really you want to keep the tread patterns and more importantly the tread depth the same across all four - but at worst keep it the same across the axle.
In the case of the pattern It's not really down the tread pattern itself needing to be the same so much as it is about keeping the grip profile the same across the tires and same brand, same model is the ...
Pros and cons of an AWD transaxle vs a separate transfer case:
more compact system
less moving parts in general
less parasitic drag
allows the car to be lower to the ground
harder to repair
transaxle is not cost effective to use in a 2wd car (a lesser model of the same car for instance)
less flexibility ...