I've used plastic zips in lighter weight items for decades - but always a name brand. Haven't looked up the style, but if it zips in back, I'd use plastic. If it zips in front and your tummy may place extra stress on it due to styling and how you prefer the fit, then I'd use metal.
I've used name brand jeans zips for years for all of my trousers, after a basic metal one failed shortly after I completed the garment. But on the occasion I make a back zip I always use plastic as I find them more comfortable. FWIW