If you are interested in developing scripts for Windows than a technology developed specifically for Windows, like the PowerShell, is a sensible choice.
While groovy will probably allow you achieve your goal I don't think there is a particular advantage that it could offer.
Saying that, you have to consider familiarity factor and learning curve. You say developers have experience with Java and Groovy in which case running ant tasks via the groovy DSL can prove easier than learning PowerShell.
Regardless, consider developing your scripts using the TDD approach. Groovy offers plenty of frameworks (try Spock for example) while there are also ways to achieve that for PowerShell.