i think that there is no difference between them , but it depend on how you are going to use them. like in these situations:
A son making a confession to his mother : I'm the one who broke the window last year !
The mother : it's okay ! it's not a big deal.
in another situation:
a wife to her husband: do you prefer me to cook the dinner tonight or shall i ask the maid to cook it ?
the husband : there is no a big deal whether you gonna cook the dinner or the housemaid .
in both situation they have the same meaning that it's "not important"