I agree with everything you've said here, including that in the two door situation the decision could go the other way if you had more information about the ratio of the utilities. Still, it seems to me that what I said is right in this way: if you are given no other information except as stated, you should choose the second door, because your best estimate of the ratios in question will be 1-1. But if you have some other evidence regarding the ratios, or if they are otherwise specified in the problem, your argument is correct.