http://justice4assange.com/US-Extradition.html#WUKJA
Some critical voices claim that the UK-US extradition treaty is more permissive than the Sweden-US extradition treaty. Extradition to the US, they claim, would be simpler from the UK than from Sweden. This argument fails on several points:
The UK’s extradition treaty does not have the temporary surrender (’conditional release’) clause. The UK’s judicial review process, while far from perfect, has a number of practical review mechanisms. The nearest equivalent case, of Gary McKinnon - a UK citizen who has been charged for hacking US military systems - has been opposed in the courts for 8 years.
Public opinion and the media (to a greater extent) are more sympathetic to Julian Assange in the UK than in Sweden. Public pressure could draw out the process of extradition to the United States in the UK. In Sweden the media climate is hostile (see Media climate in Sweden) due to the sex allegations. Public outcry would be significantly weaker and therefore less likely to stand in the way of a strategically convenient extradition.
In the UK, Julian Assange is better able to defend himself, muster support and understand the legal procedures against him. In Sweden on the other hand, the language barrier prevents him from effectively challenging the actions against.
The UK is politically better positioned to withstand pressure from the United States than Sweden. Sweden is a small country of nine million people close to Russia. It has grown increasingly dependent on the United States. In recent years Sweden has complied with directives from the United States in a manner that has not been scrutinised by Parliament, as has been revealed by the disclosed diplomatic cables (see Political Interference).