Both $CO_2$ and $SO_2$ react with water to form acids, but $SO_2$ produces a stronger acid.
$$CO_2(g) + H_2O(l) ⇌ H_2CO_3(aq)$$
$$H_2CO_3(aq) ⇌ H^+(aq) + HCO_3^-(aq); Ka_1 = 4.5\times10^{-7}$$
$$SO_2(g) + H_2O(l) \rightarrow H^+(aq) + HSO_3^-(aq)$$
$$HSO_3^-(aq) \rightarrow H^+(aq) + SO_3^{-2}(aq); Ka_2 = 1.2 \times 10^{-2}$$
The stronger acidity of sulfurous acid ($H_2SO_3$) compared to carbonic acid ($H_2CO_3$) is due to the higher stability of the sulfate ion ($SO_3^{-2}$).
Therefore, introducing an equal amount of $SO_2$ into water will result in a lower pH than introducing an equal amount of $CO_2$.
