There are two major factors to keep in mind. First of all, the more cooked the potatoes, the higher their glycemic index. Oddly, with both the potatoes and the pasta, the less cooked they are, the less glycemic they are. Therefore, you need to find a very fine balance between cooking them just right, and not overcooking them. That is the reason why the Italians usually cook their pasta al-dente - just a bit undercooked.
Secondly, most people add lots of stuff to the potatoes when they cook the mashed potatoes - milk, salt....condiments.