I think there are differences depends on the person, the Japanese language is ambiguous (vague / fazzy). In my opinion, "this time" is 今回 or 今, and 今度 means "next time" or sometimes it also means "some day" depends on the situation, however, it's mainly used for "some day" like greeting, it doesn't have the specific day. I would use 次回 or この次 for next time when I /we have the specific day. Ex.) 今回のテストは簡単だった。This time, the exam was easy. 今度の / 次の / 次回の テストは難しいでしょう。The next exam will be difficult. 今度の(=次の)授業ではひらがなの練習をしましょう。Next time (In the next lesson), let's practice Hiragana. 今度いっしょに食事をしましょうよ。Why don't we eat out together next time / some day? I hope this helps you.

It depends on contexts. Here are examples. http://dictionary.goo.ne.jp/leaf/je2/27652/m0u/%E4%BB%8A%E5%BA%A6/

@Joe I don't know if you watch Naruto, but it's the phrase Itachi always says to Sasuke while touching his forehead lightly in the middle, who hates it - "また今度な。" It's how I like to remember it :)