Each S1 S2 S3 costs the same, you do not miss out on anything by purchasing S2 and later S3, nor does it cost any more in total. If you are contemplating buying S3, then just buy S2 now, enjoy that for a while then when S3 is released, pay the little bit extra and upgrade. No point waiting for S3 when you can play S1 and S2 now.
If you like the demo then S2 will make you happy and keep you busy for hours.