I still don't understand why Sweden is considered more likely to extradite to the US than Britain is...

In the UK an judge may make an independent determination that his 'crimes' are of a political nature and therefore not extraditable. In Sweden, there is a newish form of extradition, temporary surrender, where it is thought the decision can effectively be made by politicians instead of judges. Is he better off with a judge or a politician?

Sweden has recent form for taking legal shortcuts in this area.

I don't think many consider Sweden to be that. Maybe he is just trying to escape his accusations in Sweden?

No, they've offered several times to allow Swedish officials to conduct whatever formalities they need to in the UK. They've refused.

Sweden also has a history of dropping prisoners off at the airport where they get met by people who are DEFINITELY NOT CIA agents and disappeared.