I think white rice is cheapest and easiest to make: no peeling, no need to turn on the oven. Microwaved potatoes are pretty fast too. And couldn't you cool the baked sweet potatoes too?
As for the GI, I wouldn't worry about it too much. If you're eating the carbs post-workout, an insulin spike is good (assuming you have an otherwise healthy metabolism; ie no diabetes). I don't know if there are any guidelines quantifying how much fat will blunt a spike as a general rule since there are so many variables and so much variation between individuals. The best bet would be to test your blood glucose before and at several times after (30 min, 1 hour, 2 hour) eating to see how it affects you.