I am a redditor, so I am biased. But I will tell you the legend as I know it...

In the beginning there was 4chan.org. It was a magical place where people of all types could come and post *literally* anything they want as long as it wasn't illegal. Many a lulz were to be had with 4chan, but soon, 4chan became dark. The 4chan people love this darkness, it is their way of life. Only the strongest of the newcomers could stomach the relentless onslaught of dark humor.

One day, enough people gathered together and said, "You know, I wish I could have the lulz of 4chan, but without all the crazies". And thus reddit was born. Reddit could be likened to a great industrial city, it has nice roads, but it also has its seedy underside. You could imagine then Reddit could be Orlando, and 4chan, Miami. Eventually a rivalry broke and the 4chaners and redditors began claiming rights over the lulz they have created. They have been locked in a war ever since.

Then, along comes 9Gag. If 4chan, and reddit can agree on one thing, its that 9Gag is a worthless scumb heap only fit for infantile adults with herpes for eyes. 9Gag has come to our lands and stolen our original lulz and claims the glory for themselves.

So behold, should you choose to side with 9Gag, you will choose to side with death.

Bottom line:

4chan = hilarious, very intellectually intelligent, VERY original, but batshit insane

reddit = hilarious,very grammatically intelligent, somewhat original, but not nearly as batshit insane.

9GAG = stupid ********* who don't know how to do anything but eat their own **** and steal everyone elses.

Edit: Take note, many redditors browse 4chan, and vise versa. BUT NEVER EVER 9GAG.