Hi Lee,
Ha, a tricky question!
I had been so impressed with my previous Lexus that it was an easy decision to stay with the brand. Also, my regular drive on a daily basis has swung a bit more urban than it had been so a hybrid seemed like a great option for me. Other factors such as car tax (in Ireland on the IS220D is about €1,100 versus about €300 on the GS450H) and fuel consumption (at the moment for me is about 30mpg diesel so if I got anywhere close to 40mpg in petrol, I would be savings about €1,500 per annum) really help with the finances!
I had a GS450H for a weekend about two years ago and enjoyed every minute of it. I also had a short drive (20 mins) in a IS330H around the same time and it just fell a bit short for me. Really well finished and comfortable but the extra pull from the GS drew me in.
I did think about a third gen GS (lovely looking car) but I have a history of holding on to cars for a long time so it made sense to buy a newer model.