I also see both in different examples. As in : 今日は暑い。 本日はおめでとうございます。 Both are translated as "Today", but when and how should I use either one? Can we say : 本日はあつい。 And 今日はおめでとうございます。?
すぐ, and especially 今すぐ, is my habitual way to express "soon", or "right after this". I know that じき means more or less the same thing, and when I hear it or see it written, I get the general meaning ...