Sliced bread contains more preservatives

Although there is an American saying 'the best thing since sliced bread', the fact is that pre-slicing bread simply explores more surfaces to airborne bacteria. As a result, such industrially produced bread requires more spoilage retardants so as not to go mouldy too quick.

A whole loaf moreover allows you to slice as thin or thick as required.

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I'm conducting an experiment with my food. I have one loaf of pre-sliced bread and it has been exposed to air for almost three days now, and it's still good! I don't think that's really normal, or healthy.

One would have to use two loaves of the same bread -- one pre-sliced and the other whole. Depending on the humidity in the air such bread may simply go stale (become rock hard) without going mouldy.