I have an IS300 but liked the look of the newer IS, so I took an IS250 for a test drive, I was really disappointed from a performance point of view, it felt totally gutless, just about seemed to get going if I really revved it round. My IS300 felt incredible on the way home.
Think I'd rather have the GS300, I know it's a lot bigger but on paper the performance looks better.
I have briefly looked at the GS. I personally think the IS looks better, all variants (other than the hybrid) have a tax brand bracket in the >£400/yr mark & there's only a couple under £6k, and most are £7k+ which is more than I was wanting to spend (especially then having the more expensive road tax)