S1 will give you the XRT - the car most commonly used for drifting - and also access to the Autocross area and the extended Blackwood industrial area, and the infinite possibilities afforded by the layout function.
It won't give you access to the FZ50 (another fun drifting car) or the supremely extensive Westhill automotive playground.
But as Scawen says, there's nothing to stop you upgrading to S2 at a later date.