Whereas the recently announced PS3 Super Slim ("Slim-Slim"?) will come with a 250GB and 500GB SKUs, the Wii U will only come in 8GB and 32GB models. To make up for this, Nintendo will allow easy attachment to external hard drives.
John Koller, SCEA VP of Marketing, has stated in Engadget that the main reason for the PS3's large internal hard drives is due to consumer demand, "to make sure, out of the box, that there is an option for them to be able to download that content". This is in particular why the 12GB Flash-based system available in Europe and Hong Kong is not coming to America, though it should be mentioned that PS3 Slim-Slim will support USB storage expansion and internal HDD replacement.
Nintendo America CEO Reggie Fils-Aime, however, believes that "the cost of that type of [external[ storgae is plummeting". Instead of forcing consumers to purchase a large hard drive with their console, they will "let the consumer buy as much as they want, as cheaply as they want".
Now, several notes here. First off, Sony is sending a mixed message when you consider that the Vita didn't come with large internal stoarge at all, something that would have been great for downloading PSN's offerings. That said, the PSN catalogue is much more robust that Nintendo eShop's, which would partially explain why it wouldn't make much sense for Nintendo to create a Wii U with a 100GB+ storage.