The UK's extradition treaty with the US is felt by many to be skewed in favour of the US [1]. I doubt Sweden's is, but I can't find any information on it. If I felt like attributing malice to the US then I'd say it would be in their interest for Assange to remain in the UK where he would be easier to extradite.
In fact, the opposite was true: before the revised extradition treaty, the evidentiary standard for extradition to the UK was lower than the standard for extradition to the US. The revised treaty more or less equalized the standards.
The treaty was controversial in part because it did lower the evidentiary standard required to extradite to the US (in other words, it did make it easier to extradite to the US). What the controversy didn't recognize was that the former treaty had been unbalanced.
You are correct about the evidentiary standard being roughly equalised‡, and the Baker review confirmed this.
A legitimate concern though is about forum and jurisdiction. The treaty currently allows the US to request extradition for acts committed in the UK that are not a crime in the UK, but there is no corresponding entitlement for the UK to make such request to the US. Many people in the UK feel this is an encroachment on UK sovereignty, and not something that the US would itself offer to another country.
(‡ shame it had to be rounded down, rather than up)
[1] http://en.wikipedia.org/wiki/Extradition_Act_2003#Controvers...