Maybe single-player wise, Harvest Moon has more content than Animal Crossing, but when you go online with friends...man Animal Crossing is amazing. There's so much stuff you can do, and creativity can lead to a lot. There is just way too much fun to be found online.
And as one dude here said about the interior design.... animal crossign IS kinda like that. New Leaf coming out is adding a lot more customizable options to decorate your home AND your town itself, and while its fun to create a town and house that is of your own design and has its own feeling and tone to it, its even more fun to visit everyone else's unique towns and homes.
The character customization is great too