the 1995 BMW M3 come with a s50 motor which is 3.0 litre which is equiped with 240bhp/225 ft tourque.(stock)

the 1996-1999 BMW M3 comes equipped with the s52 which is a 3.2 ltr motor that also has 240 Bhp/236 ft toruque.(stock)

The reason 96-99 have a 3.2 litre motor is because they are not oBD1. the made teh OBD2 motor a 3.2 litre so the car wuold have less horsepower then there previous year. The main Perk is that the car is obd1, you will have not worry about the different stage softwares that u will with the 96-99. The only time u will have to get special software for my car is when u have to go in the motor i.e. cams, pistons, Force induction etc....

Hope this answers your question