Saka as in 酒場{さかば} or zaka as in 居酒屋{いざかや}
Sake as in 酒{さけ}
The difference between さか and ざか is just one of rendaku, which I think is well discussed here: Rules or criteria for 連濁: Voiced or unvoiced syllables in compound words
The reason that 酒{さけ} becomes 酒{さか} in some cases is not really directly related to rendaku(連濁)rather this is called ten'on(転音).
Just as a short answer, some words, when part of a compound word, will have a change in the vowel sound of the final syllable/mora.
This change seems to sometimes carry the implication of「何々の為の何々」.
え turning into あ:
い turning into う
い turning into お
You can see other examples here: http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q12109076097
The best Japanese reference I can find that really concisely outlines what's going on with さけ・ざけ・さか・ざか is this one here: http://www.nihongokyoshi.co.jp/manbow/manbow.php?id=845&TAB=2