I have an SSRS report that has a cell which I need to show the value of a Dataset item unless a parameter is selected, in which case I need to display a sub-report in that cell.
I tried setting the sub-report's visibility but then it never displays the expression since it takes over the entire cell. It's like it is either one way or the other.
If you put a subreport in a cell then you can't optionally display something else in that cell.
However from your comment you're trying to display values from two different datasets based on a condition, and you should be able to do this with an expression. Assuming there is some field in the table that can be used to relate to either dataset, then you might be able to use the lookup() function to get the relevant value, e.g. a code outline for this:
Iif(some_condition, lookup(value1 in datasetA), lookup(value1 in datasetB))