Sometimes it is not as straightforward as carb comparisons. There are portion comparisons and the weight made up for porridge, weight for weight made up it may be different compared to a serving of museli.
Also, some of us can tolerate some forms of carbs, but not others. That is why meter testing before and 2 hours after eating is so vital. it helps us to find out hwo we are affected as individuals.
For example, I cannot eat porridge, museli, cereal, rice or more than 1 tablespoon of pasta at a serving. But i can eat potatoes and wholemeal bread. Someone else may have the opposite results.