Depends,
what is you version of 3ds/n3ds?
if it is 9.3 or above, then you need sky3ds, gateway won't work at all.
9.2 or below, then both gateway and sky3ds work.
With gateway, you can do more thing than sky3ds, but I don't use them a lot. For example, you can use it to decrypt 3ds rom, so you can play pokemon hacked rom for example.
Gateway is also region free, so you can play game from another region that aren't released in your region.
There is also the game menu. By pressing the select button on the home menu, you can choose the game you want to play from a menu that pops up.
However, sky3ds is way more restricted.
Sky3ds has a blue/red button on the card itself. Depending on the quality of your microsd, it can takes time to choose your game.
You press the button, then wait few seconds so the sky3ds load the next game on the list. So if you want to play the 5th game on the list, you press the button, wait 2-10 seconds, press the button again, wait 2-10 seconds, etc..
So, with the sky3ds, it can takes time before to play your game.
However, with Sky3ds, you can update your 3ds to the latest version since it hasn't been patched by nintendo yet (and probably never will)
There are advantage on both flashcard, so, it's your choice