I'd go with one long string of words and phrases with no commas. Often times one or more words end up getting repeated many times over. Say the page is all about golf communities. The keyword tag may end up being texas golf community nebraska golf community amarillo golf community................. In this case the main keyword will get repeated numerous time in order to include the main keyword with all the different regional modifiers.
There have never been any problems we've experienced by doing things this way.
Assume you are talking "meta-keywords" in which case don't worry either way, as these days they have seem to have little relevance for SE results (except maybe Inktomi which seems to look there if it can't find the word in the main text).
Even so avoid "keyword stuffing" as a lot of surfers may look at your code and if they see this it immediately flashed a red signal SPAMMER.!
I've always thought that the inclusion of commas indicates the usage of a phrase... this blue widget as a phase is inputted into the meta field... rather than having a blue and widget meta tag. Am I wrong?