Of course it's a matter of personal preference but the big difference is that the GS is going to be either RWD or AWD. The 3 cars I had before my GS were all FWD so I was a bit apprehensive going to a RWD. Now, I'd never go back. And remember, I live in New York where traction counts at least 4 months out of the year. For performance, the handling of a RWD is far superior to a FWD. There's a reason that nearly all of your top end luxery cars, sports cars, and cop cars are RWD. They handle so much better. But while RWD rewards a good driver, it punishes a bad driver. Too many punks think that being a good driver means you can steer a car and push hard on the accelorator. It's all about being intelligent and knowing the limits of your car. Do the test drives but in a nutshell I would say that if you want to hop in, sit back, push the gas and go, the ES is a great car. If you want to be more actively involved in the handling of the car, the GS is awesome. You really can't go wrong.
Hope this helped.