I test driven the 323i about 2 weeks ago.
I am pretty impressed with the handling of the car round the 99 bend and it's pretty composed when being pushed. The suspension setup, according to the Sales is that the 323 is running on sports suspension and it really feels pretty good.
On the move, the 177hp is pretty sufficient but i feel the car sounds a bit strained. That might be the engine is still tight and may loosen up overtime or with a simple oil change.
All BMW are RWD and all 6 cyc has got cruise control as standard, as with cabin floor light and rear reading light.
The 323 is a 6 cyclinder and BMW is known for it's creamy 6 while the 320i is a 4 potter, which makes it more economical to drive. The 323i is a 2.5L and the 320i is a 2.0L. The difference in tax is close to 800 buxs a year, consumption of a 6 cyc will def be higher on a 6 cyc as well
Know your own needs and choose from there.