I have a K&N filter in my 1989 900i and also in my 1990 9k carlsson, and an ITG filter in my 81 turbo 8V sedan. The 16V air filter for a classic 900 is longer than the 8V version so check that you get the correct size.
The main thing with any non-paper filter is that they need maintenance, so once a year they need checking and if they're dirty, a complete clean and re-oil.
The K&N filters are different construction to the ITG ones, though I haven't noticed any appreciable difference between the two.
Craig.