Same Features,
Support 4.1~4.5 3DS/3DS XL console system
Support multiROM
Game cart backup
EmuNand 7.x
official carts region free
Support Eshop
Support SaveEditor
The Difference
Obviously, Gateway and MT Card comes from different official site, produced by different team then it is sure that they have different hardware and software. Second, gateway 3ds can dump 3ds roms from the retail 3ds game cart, but mt card can’t. The features of them are the same expect this. But the price for gateway 3ds is much more expensive than mt card. Maybe mentioned above, gateway 3ds has one more feature, but actually this feature is not that important,because we don’t use it too much, we can just download the 3ds games and don’t need to dump them from the retail cart, in the other hand, if we already have the retail cart, why we need to play it with the gateway 3ds.
The conclusion,
Both of them are very good cards and update regularly, so you can buy both of them, though gateway 3ds is more popular because it is the first one, i still recommend buy mt card for its cheaper price but same features.