This is a really easy quetion for anyone who has studies Asian languages.. I am surprised how many people have no idea.
Korean has only 13 vowels and consanants. You can learn to read it in about 30 minutes. It is simple memorization, just as you learned to spell as a child.
Writing Korean becomes more of a challenge, but again, you memorize the 13 vowels and consanants, with a few changes here and there. Your written word would be far from perfect, but people would get what you were trying to say.
From there, the basic sentence structure is similar to the other languages.
However, learning Japanese or any of the Chinese dialects would require years of studying before you would begin to understand the kanji and so forth. (the written symbols)
If you already know Chinese, learning the written Japanese would be easier for you than a westerner, and vice versa, but the spoken work would stil be just as difficult.
Even for a native speaker, knowing all of the kanji is nearly imposible.
At least in Korean, you could sound it out from the get-go.
Most who study Japanese and Chinesse start out with the romanization of hte language first, and after about a year of language, move into the basics of the written.
As far as language utilization, there is no doubt in today's world that a Chinese dialect would be the most useful for your future. but I find Korean the most fun.