This is a tricky question. Ideally if you are close to the same level in skill and have good communication via skype of curse or something, it should provide a slight advantage over the long haul.
Problem is that is almost never the case. In a DUO there is almost always a weaker link. The game pairs you with players from the higher persons ELO, and now you almost always have 1 player that is below par on your team every game. That means someone has to carry harder than normal to win, and even if they manage to pull the person up over time into their ELO bracket...its artificial. If they try and play on their own they will likely slide down the ladder very fast.
Duo queing can be fun, and is a great way to help teach someone better habits, but i think its a bit of a liability in ranked. Its never as cut and dry as it always being better or always being worse. I think the best you can do is put in some real time in normals together before you try and tackle ranked as a duo. Also remember that duos don't always get to play bot together and so your mate will have to be good enough to hold their role in every lane, not just as your dedicated support (or ADC).
Good luck, hope you guys are having fun. Having fun can have just as much impact on your game as your pairings.