The GS300's are very very nice cars...as people have said they are very roomy and more luxurious than our IS300's...but since its a pretty big car the handling and power is going to be down form your IS300...itd probably take some time to get used too driving it....
The GS is a really nice car and the SportDesign package is a beauty. The larger question is do you want a larger car? The GS is more a family sedan where the IS is a sport sedan. Also no manual available but that may not matter to you. If it was me I'd stick with an IS. The GS is just not my style. It's more a style question. Which car reflects your personality more?
Hope you can make a quick desision. Will the dealer extend your lease a few months?
I definitely wouldn't. Even if you get a killer deal, the GS is overpriced. Let's say you can steal it for $34K, that's still about $4K more than a decently equipped G35 sedan. The G will outrun, outgun and outhandle the GS, all while delivering generous room in a 4 door sedan. The only trade-off is a little less luxury in the cabin. And just like Lexus, Infiniti's reliability is excellent. After driving both, I liked the IS better than the G35, but if I had needed more room, I would have got the G. Best of all the G35 is available with a proper transmission, a 6speed manual. Only problem is the 6spd G's are hard to get deals on because they're still in short supply. No wonder, with a 0-60 time of 5.7 seconds.