It doesn't really matter. Since texting seems to be all about abbreviations the 'k' is probably the most consistent.
You may be interested to know that the Japanese word for OK is "yosh". I was once talking to a Japanese national and used "yoshi doshi". It didn't take him long to work out that meant "okey dokey". My personal contribution to Nihon-go.
And, apropos the picture with your question, you're wrong. There are only 10 kinds of people in the world: those who understand binary and those who don't.