Bore Da Pawb
Which one of these is correct? Or are they both correct but one is more natural?
I was going to ask but you didnt have time:
O’n i’n mynd i ofyn ond doedd dim amser gyda ti
OR
O’n i’n mynd i ofyn ond chest ti ddim amser
Please and thank you 
It’s definitely the first one. The second one (using cael) is used when you get or receive something, while the first one is used for having or owning (even for intangible things like time).
So: Doedd dim amser gyda fi ddoe → I didn’t have time yesterday
Ges i amser da neithiwr → I had a good time yesterday night
(And a side note: your Welsh sentences literally say “I wanted to ask …”, which is basically the same sense, but if you wanted to stick closer to the English “I was going to ask” it would be O’n i’n mynd i ofyn …)
I meant to write “mynd i” - Brain fart.
I thought as much but the “chest ti ddim” construction is used in the challenge.
Could it be the case that it is colloquially interchangeable?
No, the two structures are not interchangeable, colloquially or otherwise. If the structure is really used in this context, then this would be an error (which happen from time to time). Can you provide the rough time-stamp and the challenge where this occurs? (I am guessing you are on the Southern course because of moyn)
At about 29:45 of the downloaded old course material (Level 1 - Challenge 25 - South)
Interesting. I guess in this case you could argue that there is not much difference in “having time” or “getting time”, but usually, the meaning is quite different:
Doedd dim car gyda fi am flynyddoedd, ond ges i un mis yn ôl. → I didn’t have a car for years, but I got one last month.