Return value only supports integers, as it needs to match the value to a return code. We are considering a range of enhancements to this functionality, and I will note your request in our backlog.
In the meantime, my recommendation would be to write a python script to perform this comparison and get it to return an integer value (example exit(3)) that you've configured on a subsequent step path. Like what you would do for return value.
Run your python script using the 'Executable' step type.
If you have any other questions please let me know.