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The keyword itself, fully spelled, is searched for hundreds of thousands of times. The abbreviation, when out of context, is not recognizable at all. (unlike "FBI," for example, which is perhaps more recognizable than the fully-spelled-out "Federal Bureau of Investigation")
So should I abbreviate it to make it readable, or should I just say the hell with it and use the cumbersome complete spelling every time? Without a doubt, if I were most concerned with readability, I'd abbreviate it.
As for keywords, overdoing it is as bad as not doing it enough; if your site is readable for vistors, it's probably about right for search engines.
Keywords - or phrases - are whatever you think your potential visitors might search for. So 'both' as offered above, is right.