Nintendo is certainly keen for Wii U and 3DS owners to consider going the download route with their favourite games. While there are still some obstacles scaring some away — such as premium prices and hardware-based account data — there have been growing numbers making the move to a virtual library.
Nintendo of America, for its part, is pushing the idea for this busy shopping season, pointing consumers to a Best Buy deal that gives 20% off eShop card purchases, essentially acting as a discount on potential downloads.
You can see the options on Best Buy's eShop card discount page, with the offer expiring soon.
You'll have to wait a short spell before redeeming them as the eShop platforms are currently down for maintenance. It's seems highly likely that you'll be able to share those funds between your 3DS and Wii U using a Nintendo Network ID, if the promised 3DS system and eShop updates arrive following this maintenance; we'll know for sure very soon.