|Just because they're the same size doesn't mean they're both cakes.|
If it was for a dessert cake that could be kept refrigerated until close to the time to serve it, sure, have at it. But not for a wedding cake.
But then I see "mousse" offered on a lot of other baker's websites, and I have to wonder what's going on.
I have a sneaking suspicion that someone somewhere is selling a sleeved filling that they're trying to pass off as "mousse." Hey, they do it with so-called "custard," so why not with a mousse?
Then I did a search for mousse recipes to see if I could clarify this. Turns out that a lot of people are under the impression that cream cheese, butter, and cooked egg yolks are ingredients in mousse. Uh, no, not really.
A mousse is made from whipping cream, eggs, and the flavorings. Sometimes you stabilize it with gelatin if the flavoring requires that, but there's no cream cheese or butter in a real mousse.
It seems to me that people have started calling ANYTHING that's whipped at any point in the process of making it "mousse." I'm sorry, but whipping pudding then serving it in individual cups doesn't make it a mousse.
Custards are cooked, which makes them more stable, and that also seems to be what a lot of recipes are calling mousses.
I even found one recipe that was for ganache, but it was whipped and served in cups. That's eating truffle filling, but it's not a mousse.
I actually do suggest truffle fillings to clients, because that's usually the kind of thing they're looking for, It's nowhere near as light as a real mousse would be, but it has a similar mouth feel, so people are happy with it. Not to mention that it stands up better at room temperature as a wedding cake sits out on display for hours before it's cut.
So be clear when clients ask you for a mousse. Most people don't really know what a mousse is, but they're looking for a specific consistency. If you can figure out what that is, you'll be able to offer them something that will make them happy, but will also not dissolve inside the cake.