A cream pie is any pie with a custard base (egg, milk, flour). A meringue pie is any pie with a meringue topping.
The difference between a lemon cream pie and a lemon meringue is one uses a whipped cream topping (or none at all) and a lemon meringue uses a whipped egg topping.
It is important to note while meringues are often used for cream pies, they are not exclusive - if you were to make a standard blueberry pie with a meringue topping then it would be a blueberry meringue pie.
Personally, on the rich/sweet scale, I'd say they're pretty similar. You can add less sugar to both meringue and heavy whipping cream to make them less sweet though. Meringue has a more eggy taste to it so you can tell what you are eating pretty quickly.