First of all - passed, not past
I don't see any difference in meaning. However
it's the case that... sounds odd to me. I've fed both of them into a concordancer and got 5 instances per 2m words for "
it's true that" and none for "
it's the case that"
I then tried other variations and found it did come up - again 5 instances per 2m words when combined with modal verbs and adverbs :
it is also THE CASE THAT, until recently, the private bond market was...
it will usually be THE CASE THAT the cumulative effect of...
it's no longer THE CASE THAT the, that the owner of...
it is often THE CASE THAT reeds, rushes and irises are expected to
I seems to be the adverb which is crucial to making it sound natural.
it's true that... on the other hand seems to be used predominantly on its own, though the adverbs
quite/undoubtedly/certainly/probably also occur, as do connectives such as
however/none the less/also and adverbs of frequenccy -
usually/always. Doing a wider search of 16 million words I found 37 occurrences of
It is true that... as opposed to 13 with an adverb incorporated - eg:
It is probably TRUE THAT in most online...
it's quite TRUE THAT erm, it's rare...
it is usually, though not always, TRUE THAT long-haul shipment by rail
it is also TRUE THAT Braque was the...
So : I'd say the normal use is :
It's true that - on it's own or with an adverb incorporated
It's the case that - usually only with and adverb incorporated.