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GS and IS are very simliar cars sharing lots of parts from the factory anyways, I think you've made a good decision keeping your IS. I have an IS350 and thought about a GS before I bought my car, to me IS350 and GS350 are the same car just in different sizes.
They are not necessarily the same, the GS350 can be had with AWD but on the IS you have to get the smaller engine IS250 to get AWD.