the main downside to REST is that the REST web services is mostly still in beta (though a number of additional records are coming out of beta in 2021.1) and i suppose it depends on your integration/your risk tolerance, but i wouldn't want to build my integration upon something very recently out of beta/still in beta. so if it can be done with RESTlets, that lets you use REST. but if you want to use web services, then i'd be wary about switching to REST right now (maybe by the end of the year?). or maybe someone can share if they're already using it in beta, if they've run into any issues - it's possible it works fine, but my experience w/ other netsuite beta features is that i usually give it 12 months after coming out of beta before it is reliable.