Just went through the same delema...
Have been living OS for the last 9 months without a car, spent all that time looking at new models etc and had made up my mind that a change was due upon my return to OZ.
My IS200 was in the shop getting a major service done when I returned, so still not being able to drive convinced me a new "powerful" car was needed, found a nice low KM 540i with full Schnitzer kit and was ready to start talking dollars when I picked up my IS200... just could not see myself switching brands once I took my car for a drive again.. but what to do, needed to buy a new car ( just had too because I made my mind up) but didn't want to get rid of IS200 but wanted more power... 2 days later the 200 was gone replaced by gorgeous Pearl White IS300.. and wow.. took it for a little 4000km drive in it's first week ( the car had 20000km on it) and loved every minute of it, the power.. the comfort.. the lights.. the pop up washers whilst very gimmicky never cease to crack people up.. only thing I would say as a criticism is that it is not as "chuckable" as the IS200, but I am sure some suspension work will fix that.. still a great handling car though.. I say go for it!!