5V, because that chip works only on 5V and outputs 5V. It would be sa great chip for level shifting 3.3 to 5V, but will not work for level shifting 5V to 3.3V.
No it would burn up the 3.3V device if you use a 5V supply or would not work if you use a 3.3V supply,
Even if the chip did work at 3.3V, the 3.3V output is generally not enough for a 5V MCU. But also if you power the chip with 3.3V supply, it cannot tolerate 5V on the input.