I grew out of my 4GB SD after a few months. Now I use a 16GB. Perfect for my backups, since I have another 3DS, and I like to install my carts on my n3DS so I can play the games multiplayer with my brother.
https://www.amazon.com/SanDisk-Ultra-200GB-Memory-SDSDQUAN-200G-G4A/dp/B00V62XBQQ
Psvita, 3ds, etc. This is your best value per mb. The ,speeds are limited on both devices and you will not get a card with higher w/r rates regardless of the higher card specs. Individual cards specs are limited by the device hardware specs, not the cards specs. Things are always limited to device device hardware. Aa both devices are capped on read write speed, through research his device will not be matched on speed or price on any device.