The subscriptions are priced the same as what Nintendo is offering so it doesn’t make a difference who you buy it from, unless you have a preference for a particular retailer. This means that if you are interested in the 3 month membership, it will be priced at $7.99, while the 12 month membership will retail for $19.99.
Unfortunately it doesn’t seem that neither retailer is selling the 1 month membership so if you want that, you might have to go to Nintendo directly (it is priced at $3.99 a month). Just last month Amazon also started to sell the online subscription so you can also opt to use Amazon if you want.
The Nintendo Switch online service is required for those who wish to play their Switch games online with other players. If your Switch play is mostly single-player and local co-op games then maybe this won’t be necessary, but at $19.99 a year, it is admittedly a lot cheaper than what the competition is offering out there.