If your goal is to carry around as little as possible, you want a Sky3DS. Gateway requires an extra cartridge in order to work on the New 3DS.
Do note that, since it sounds like you're trying to do everything legitimately (high five!), you'll need a way to actually create backups of your game carts. This can be done with the Gateway menu, but while you can get to the Gateway Menu without actually owning a Gateway, you will need either a spare Old 3DS with firmware 9.2 or below, or—if your New 3DS has firmware 9.2 or below—a copy of Cubic Ninja*.
Also, while the Sky3DS is a good way to avoid carrying around game carts that you already have, I recommend also buying any future games on the eShop. Having too many games on the Sky3DS can be annoying, as it requires you to press the blue button multiple times to select the right title.
*I'm not mentioning the Ocarina of Time exploit, because you cannot transfer save data without the Gateway menu, and if he already has the Gateway menu, he wouldn't need Ocarina of Time.