It’s not really possible to a/b those, as they run different networks. Standard Shopping is search only while Smart Shopping adds GDN ads into the mix both for new and returning users. Smart Shopping will always take priority as in if both campaigns are eligible and have same result for one auction, Smart will show. They don’t cannibalize if run on the same account – only one campaign enters final auction, so feel free to run them alongside. You can try to evaluate performance on a pre post basis.