Obviously this has been posted numerous of times before but I think it's worth mentioning again given that the launch of Nintendo Switch is imminent.
It has been confirmed that eShop credit can be used across 3DS/Wii U and Switch.
At £22.79 it's roughly about 9% discount off the original price, so you should take this into account when you buy game download code from other merchants(e.g. Amazon) and most likely it would be cheaper to buy from Nintendo eShop instead (if you don't mind having some leftover credit for future purchase)
UPDATE: You can now merge your 3DS/Wii U eShop funds with your Nintendo Network Account.: https://ec.nintendo.com/my/#/