This can also be solved using the addition rule for probability, $P(A \cup B) = P(A) + P(B) - P(A \cap B)$, where event $A$ is drawing the card 'E' and event $B$ is drawing the card 'S'. Since there is only one card for each letter out of 26 total cards, $P(A) = \frac{1}{26}$ and $P(B) = \frac{1}{26}$.
A single card cannot simultaneously show both 'E' and 'S', so the events cannot occur together, which makes them mutually exclusive and gives $P(A \cap B) = 0$. Plugging into the formula:
\[ P(A \cup B) = \frac{1}{26} + \frac{1}{26} - 0 = \frac{2}{26} \]This confirms the same result reached by direct counting.
\[\boxed{P(E \text{ or } S) = \frac{2}{26}}\]