personal comment,
I am using 520i, it actually depend what u want, if you are a hardcore driver, even 530 is not enough for you. Frankly say, 520 is not sluggish and underpower. Initially i taught so but until i know how to control and drive it,no regret. My only regret is, the upgrade space is smaller compare to others. What i mean is, when i need power to chase down another car, i would not have the pleasure to put it on "D" mode only and in diffrent timing i would need to change to "DS" or even manual.
If you wish to invest, i would recommend to go for 530D, the fuel comsumption is a surprise. However as owner for 520, it is a good car especially u r looking for comfort and if u understand it, it can be a fast car, it depends on driver not the car.