I would have thought it depends why you want to know.
- If you want to know how many carbs you have eaten, it is easier to use the carbs per portion/slice.
- If you want to compare items to see which is the "better", it is easier to use the carbs per 100g because a portion is not always the same.
So, if you want to compare different packs of bread to find which has the lowest carbs in it, look at carbs per 100g. Once you have decided which bread, you can track how many carbs you have eaten by adding up the carbs per slice.