If you have a 3.3v that is the system voltage – a digital input at 3.3v will read high, and of course 0v will read low. The I2c and all of it’s output is at 3.3v – and if you have sensors that run at 3.3v (a lot of delicate ones do!) then you can plug them straight in, and they’ll be fine. On the other hand, the 5v version runs at 5v, so for it HIGH means 5v, not 3.3v. So if you plug your 3.3v sensors straight into a 5v arduino, you’re likely to toast them and make the magic blue smoke come out. The advantages of the 3.3v are that it uses less power and can talk directly to 3.3v sensors, and the advantage of the 5v is it has a faster clock frequency, an can talk straight to a 5v sensor.
I hope that helps…!