The experiment itself is pretty straight forward, put both in a bowl and see which spoils faster. Testing with your nose every interval (~30min) and record the results as you go in a log book. The tricky part of this experiment is the 'why'.
Typically experiments start with a testable hypothesis. In this case it could be something like 'Whipped cream will spoil faster because of ____' Where the blank could be anything like "it has more air in it", or "the proteins in milk provide a better environment for bacteria start breaking it down". You then design the experiment to test the hypothesis.
The problem with this is that you have the experiment first, and are trying to come with a hypothesis- and I cannot think of a hypothesis which would result in these two substances being tested for spoilage.
You could try whipped cream vs un-whipped cream, that's more an Apples to Apples experiment. The hypothesis could be that air lets the cream cool faster, and warmer environments promote bacteria growth. Your testing could then use a thermometer to see if heat does play a factor, or if its just that the air allows for more bacteria to mix into the cream.