I am a total fan of a twin-turbo upgrade and I really think that a single turbo upgrade takes most of the N54 qualities (but adds power).
If you can find the space to fit the two turbos into the engine bay (and have the money), I would say do it, and go for the smaller AR, in order to keep the engine torque characteristics.
I made a software program with which I can do calculations on theoretical maximum power obtainable from a turbo setup, given the turbine efficiency maps:
http://www.e90post.com/forums/showthread.php?t=698432
So if you have a target power in mind, you can choose the turbos based on that power. I know that GT28 are the smallest Garrett turbos with ball-bearing shafts but maybe even smaller turbos may be sufficient.
If you are really serious about this, I can run some numbers for your turbo setup (it does take like an hour to input the data though
)