As a general rule, brands are cheaper in their origin country, e.g Chanel in France, McQueen in the UK, Marc Jacobs in the U.S etc. So it would depend on what designer you're looking at, otherwise I'd definitely recommend you price things online first. I'd also check availability of the designer in both those cities - for instance, Rick Owens or Chloé are a lot easier to get in London, but if you want Fendi, you'll find a better selection in Rome.