The major difference between the iOS store and the upcoming Mac App Store will be the volume of available apps. The Mac App Store will likely result in a larger volume of available apps than there are today, but compared to the sheer numbers on the iOS store, it will still be a small amount. This will perhaps make it easier for newcomers, but as Davide says, it's down to the quality of your app and making something that people actually want to have.
There's a chance to make money in either store, but it won't be achieved by making a better version of an existing app, it will be by making something that no one's seen before.