I think they are interchangeble. i don't see any difference between em. 你就算失败了，也不要灰心。 你就是失败了，也不要灰心。 你就算是失败了，也不要灰心。

“就算。。。也”表示 假设程度重 “就是。。。也” 表示假设程度轻 但是真正用起来 两者没有差别 都是假设某种情况。

你就是个不懂事的孩子(强调事实) 你就算有1000万，我也不会嫁给你(假设是这样)

the first is more formal, the second more commonly heard in conversation, and more often used by people from China.(since i'm from Taiwan)

practically, the two phrases are alernative and Interchangeable strictly, the difference is on the characters suan and shi, but you dont want them differed unless you get bored and wanna be fuss