Both formulas are correct and yield the same result. See Ohm's Law.
Having said that, in many cases one should take into account parasitic parameters such as stray capacitance, lead inductance, resistor inductance and distributed capacitance, etc. This is because a resistor isn't just a resistor any more if the frequency is high.
But for dc or slowly variying stimuli, any of the formulas will suffice.
If the resistor is nonlinear, a graphical method can work, or a wattmeter.