It all depends.
If you're doing a low-carb program, then a baked potato is probably not on your plan and can therefore slow you down by affecting your ketosis. If you're sensitive to carbs, then the potato can likewise slow you down; there are a lot of folks who find that carbs either make them feel like crap or slow their weight loss.
If you're counting calories and have no trouble metabolizing carbs, then a plain baked potato is a great low-calorie choice--and so is the chicken, so eat and enjoy them both.
Grilled chicken is awesome, salad is awesome, baked potatoes are awesome--but not for everyone and not for all dietary plans. None of these things will slow your weight loss unless you're carb-sensitive, eating gigantic
portions, or drowning everything in calorie-dense toppings.