Before I bought IS I drove a used 1998 GS 400. Fast as hell but seemed too big and unwielding around corners and the leather was slippery unlike the escaine. Very luxerious inside but not "fun" to drive like IS.
I say get the IS, no worries with that. I bought a used car once, will never do it again. It had some major problems a few months down the road that cost major $$$. Rather get a new car over any used car just for the extra piece of mind. Plus the GS already has 33k miles, kind of a turnoff imho.