Even though the Keyword Tag is less effective these days as an optimisation tool, it does make sense to construct it correctly.
Therefor, it would be best to stick to the standard, IE., META NAME="keywords". Otherwise, your keyword tag would be non-standard and thus even less likely to be parsed correctly.
Also, as to delimiters, usually people use either COMMAS or SPACES at the delimiter. The difference is that SPACES separate the string into individual words, whereas the COMMAS separate the string into phrases. EG., "one word another here" would be four distinct words, whereas "one word,another here" would be two distinct phrases. This means you need to be careful with your COMMAS to ensure you are delimiting your phrases correctly, as just one COMMA will mean phrases.
Even so, whether this would actually create any useful SEO effect these days is doubtful and so the distinction is perhaps somewhat academic.