Translation:Are those just charming promises, or are you speaking the truth?
First of all, tio never takes a -j - so it would have to be tiuj vs tio.
The difference between the two is that tio answers "what" and tiu answers "which. Another way to look is like this:
- if you can add a noun after it without changing the meaning, you probably mean tiu
"Are those (promises) just charming promises?"
Since you can stick in "promises" without changing the meaning, it has to be "tiu" here.