If you are wondering what the difference is between the Mazda 3S and the 3I, you should remember that these two are trims that are being offered. You can choose, depending on what you think is more appropriate for you. Between the two, the S is going to be more expensive.
The S comes with features that are a lot more advanced. First of all, it will come with a better engine as compared to the I. The S will have a 6-speed manual transmission while the I version will have a 5-speed manual transmission.
The S will also come with AFS. If you are not familiar with this, AFS stands for Adaptive Front Lighting System. This means that the lights will swivel according to the road conditions.