The BMW is the better driving experience and provides far superior fuel consumption (320d) and better build quality. RWD, better engines and a significant weight saving make a world of difference, though beware the M sport suspension combined with run flats which makes for a harsh ride.
Visually of course, the 3 Series saloon is about as anonymous as they come, though if you are a business user this may be an advantage depending on your field. Whilst very well finished, I find the new 3 very cramped inside and the dials and stereo facia are very dull to look at.
The 159 Ti has it licked for pure style, presence, leftfield appeal and whilst not as well built, the interiors are fantastic overall. Gotta love the TI interior - best of any car I have ever been in.
Fuel consumption on the 159 is a bit heavier than the GT 1.9 diesel due to the weight. My 1.9 159 Ti averaged around 34 overall with a lot of city, whereas my GT averaged 38. On a longer motorway trip cruising at 80 mph gave circa 45 mpg, versus 50 ish in the GT. It was just such a great place to be though and it did handle and ride beautifully for such a heavy FWD car. Beware of the cost of boots for those big 19" wheels however as the factory fit Contis can come in at circa £280 a corner. Make sure you check the inner edge of the fronts when you view. Overall I really enjoyed my 159 Ti and kind of wish I had kept it. My clients loved it too and I used to get lots of admiring comments when on business.
Drive em both and please let us know what you think. Happy car hunting - either way I am sure you will be pleased by the step up from your GT.