Often I hear different pronunciations for the word
Some people pronounce it as
(Well, it sounds like "Nics".)
Does it make any difference? Is this a different dialect? Is it good to use as "Nics"?
It's slang for nichts, as you have guessed.
I'd love to say something more but, first, I'd like to understand what is "good to use" (obviously, don't write nix it in a formal context!), and, secondly, I'm not an expert. Whence I'm pretty sure somebody will illuminate us with a better answer.
Nix is, as was already pointed out, a colloquial, informal, shorter form of nichts.
Nix does not derive from any specific dialect; rather it is present in one form or another in most dialects. There are rare exceptions like the Berlin dialect prefering nüscht
It is okay to use in very informal writings, like text messages to friends or in a chat etc. Do not use it in any kind of more official written context; even in a letter to good friends, I would spell out nichts 99 % of the time.
It is also okay to use it in most contexts when speaking. (e.g: If my supervisor asks me if I had any success in an experiment, I can hear myself answering da hab ich nix rausbekommen but if I were in a job interview, I would make sure to use nichts most of the time.)
There are people (like me) who will pronounce nix in almost every spoken context while there are others that will pronounce nichts more often; I think that this is due to speaker's preference.
"Nix" is dialectal. Dialects which have this form exist in Northern Germany and in Bavaria. Because of this wide range, it can also be used in colloquial speech as most/all Germans will understand it. It is not slang, but use of this form might have a surprise value that would stress the message of "nothing".
By the way, "ik" is not a different way of saying "ich". Actually, it is an older form that predates the High German consonant shift and exists in dialects in which this shift has not fully taken effect, i.e. in the North.
Nix is basically the up-coming new, simplified pronunciation of nichts. As it is still rather young, it's not yet considered 'proper' German but purely colloquial, so the spelling has not been updated, and so, when you find the word nichts in a written text, you can't pronounce it nix when reading it aloud. It has a status similar to it's for it is and many other similar contractions in English, except it's still a bit more informal. Nix is how it is spelled when it does occur in written German, typically in informal dialogue.
As the OP's question has yet to be fully addressed--even the accepted answer does not really do the job--allow me to point out the difference in pronunciation. To an English speaker, nix sounds like "nicks," whereas nichts is much different. The ch sound in the German does not exist in either British or American English.
It may be described as a hollow, windy sound made near the back of the throat, and if air is not passing over your tongue when you say the ch-sound, you aren't saying it correctly.
Many speakers of English mistakenly articulate the German ch as a "k" phoneme. Common examples of butchered pronunciations are the simple German words doch (mispronounced as "dock") and Ich (misspoken as "ick").
But the ch sounds neither like a "k" nor an "sh," thus nichts is not properly pronounced as either "niks" or "nishts." To get help learning the correct tongue position for the ch-sound, practice the starting sound of English words like "Hugh" or "huge," as this is probably the closest approximation to the German ch an English speaker has.