if u wanna go up the ranks it is best to do solo, unless he is in the same division as you, it is better if same rank too. As long as you guys have good comp together duo can work but I my self perfer solo.

PS when you guys duo make sure that you guys dont blame the team because in most case sernarios in duo, you guys will most likely blame others rather than blaming each other

hopes this helps and if u wanna add me my IGN:Shadehhood

I find that duo can be better in a lot of situations.
I often duo with my bronze friend (I am bronze too) so we get stuck against the same players we would otherwise be against.

Having someone that you are able to communicate with easier than a ping or "ss" helps the gameplay dramatically.
I main top lane whilst my friend mains jungle. This allows good teamwork between calling for his ganks at the right time, helping each other notice where their jungler is along with coordinating plays through our skype call.
I know you can use Curse Voice.. But let's be honest, no one ever uses it even if you send them the link.

I find that DuoQ can help you both rank up faster if you are both placed in leagues lower than your skill level. This way, if you dominate your lane but find that the rest of your team are feeding and are getting wrecked, you can count on your duoq partner to bring up the rear and push the team forward.

SoloQ can be better if you are improving at a rate higher than your teammate. Otherwise, you will be stuck with the lower league players not progressing as your skill level increases.

Overall, I pref DuoQ because I only really play games with friends. Gaming is all about playing with others for me (Besides SP games).
I only really go SoloQ if there is nobody suitable enough to play with me.

p.s Don't DuoQ with anyone that is too high or too low compared to your skill level. If you are silver 3, only duo with silver 2/3/4.
That's my advice.