They're both "Nintendo cards," but they do completely different things. One is money; the other is a membership. Here's the difference in plain terms.
A Nintendo eShop card adds funds to your account balance, which you can spend on anything in the eShop:
Think of it as a gift card or wallet top-up. Funds never expire.
A Nintendo Switch Online membership is a subscription, not credit. It unlocks features for as long as it's active:
It's sold by term — we offer 3-month and 12-month memberships.
They're not either/or. The most flexible move is to buy an eShop card and use part of the balance for a Switch Online membership and the rest for games. If all you want is online play and the classic library, buy the membership directly.
Both are delivered instantly by email — shop eShop cards or Switch Online memberships now.