it depends, the key points are who are you chatting with and what are you talking about, if they are close friends i'd like to chat with them via text message, if they are nobody, online chat is better.
I Would prefer internet because you will get lot of options to do in internet chat,where in mobile it wont be available,so obviously internet is the better option my friend...
I like the mobile text chat more then internet.. We don't need of internet connection in mobile chat.. and we can chat from anywhere without any dependency.
I would prefer internet chat to mobile text chat. Because I find that typing in mobile text is more difficult than in computer, and has less smiley to choose.