They may work out as equal syns because of the calore content but a Hex is based on nutritional value - so two weetabix will give you more fibre and less sugar content than the cheerios would, which is why you can have them as a HEALTHY extra and not have to syn it. They provide a nutritional component to the plan which Cheerios clearly do not fulfill or they would be included on the Hex list.
Saying that you choose any cereal and count it as a Hex is the equivalent of counting a Curly Wurly as a Hex because it has 6 syns. It doesn't follow.