Answer:
the Mazda 3i has a 2.0 liter engine and the 3s has a larger engine (2009 and earlier had a 2.3L and 2010 has a 2.5L 4 cylinder motor) The 3i is only available in the sedan version but the 3s is available in both the sedan and hatchback versions. Both cars also have different price ranges and different option packages. There is also a version called the "Mazda 3 speed" which has a turbocharged version of the larger engine as well as performance tuned suspension and brakes and is very fast.