Cut the sweet and white potatoes into large dice (about ¾ inch). Combine in a large saucepan with just enough water to cover. Bring to a slow boil, then lower the heat and simmer until done but still nice and firm.
When the potatoes are done, drain them in a colander and rinse under cool running water until just warm. Combine in a mixing bowl with vinaigrette; season with salt and pepper.
Line a large serving platter with the greens. Mound the potato mixture in the center.
Scatter the tomatoes and avocado around the perimeter of the salad. Top the potato mixture with a sprinkling of pumpkin seeds. Serve at once; pass around additional vinaigrette as desired.