John.....
I too had a 200 for about 9 months, but really wanted the full leather, and hid etc......I did'nt realise just what a different car you get with the 300.....
So, where do I start, well there is the obvious things like I have mention, the wonderful full leather, and it is wonderful, really fine quality like you would expect to find. The HID lights - (in true friends tradition) OOOH MY ***! What a difference - I was never impressed with the 200 lights, but hell those 300 lights are fab - would never go back to "normal" lighting. Its silly, but when you switch them on, you can hear the gas ignite, and then you get this white glow in front you - WOW - really wonderful.
E-Shift is great, I live slighty of the beacon track, winding roads down to the river, and as soon as I get off the motorway in an evening, I switch to manual - total fun (Within legal limits of course ;-) ).
The power of the car is well balanced, so the drive is good, the exhaust note is still tuneful, but not as much as the 200 - although no doubt this could be enhanced with a back box change.
So in summary -
Am I happy - O yes
Do I have any niggles - O no
Would I have another 300 - O yes (Probably a sportscross though!)
Hope this helps tip the scales :-)
Rich