The reasoning is the same in each case: the sign of the coefficient of the squared term indicates the direction the parabola opens. If it opens upward, the vertex is a minimum; if it opens downward, the vertex is a maximum.
f(x) -- the sign of the squared term is positive, so the vertex is a <em>minimum</em>.
g(x) -- the sign of the squared term is negative, so the vertex is a <em>maximum</em>.