When would you use

$\displaystyle \frac{d}{dx}(a^{u(x)}) = (lna)(a^{u(x)})(\frac{du}{dx})$

and

$\displaystyle \frac{d}{dx}(a^{u(x)}) = a^{u}(\frac{du}{dx})$?

a = constant

Aren't they equivalent to one another? Would I get the same result using either one?