Ranking apart, the population of US is 295,734,134 and that of UK is 60,441,457. So, the population of US being around 5 times that of Britain, the number of universities in US as compared to that of UK is very much justified. As far as graduate studies is concerned US and Canadian system has mandatory coursework to be taken while British system doesn't. Personally, I go for North American system for graduate studies. Good ranking of your institute helps but is not a soul formula for success. Thats my dui kaudi ko kura.