A few years ago, prices were essentially the same, since the manufacturer set a "minimum retail price". However, after the Competition Ordinance came into effect in Dec 2015, there is now more difference in price.
Broadway/Fortress are owned by large conglomerates who need to pay for the sales staff, rent, management and shareholders.
If you're looking on HK Island, prices are going to be about the same. And if it's for a new model, prices are unlikely to vary much.