LOL...this type of questions have been asked so many times before. Best is test drive n decide yrself.
On forums like this it draws a lot of fanboys, car salesman in disguise as owners of these cars, etc.
All I can say is if BMW sell the 530i at GS300's price, I will grab one. There is so much price disparity between them and each time they are compared, the usual points are debated.
I will buy a car based on my driving style and find a car that matches that. Mind you I sat in friend's GS300 before. I felt super quiet and I like it as a passenger, not sure if I will get bored quickly if I was the daily driver. Its me only, and the only thing I didnt quite like is its shape resembling the Nissan cefiro somehow from some angles. Like the Hifi in it though.