Answer
$\Delta h_{argon} = -26.02\ kJ/kg$
The change in enthalpy would be different because the two substances have different $c_p$s.
Work Step by Step
Assuming $c_{p,avg} \approx c_{p,300K}$:
$\Delta h = c_p\Delta T$
For argon ($c_p = 0.5203\ kJ/kg.K $) and with $\Delta T = -50\ K$:
$\Delta h = -26.02\ kJ/kg$
For neon ($c_p = 1.0299\ kJ/kg.K $):
$\Delta h = -51.50\ kJ/kg$
