> It's easier to extra judicially grab at least non citizens from Sweden.
Even if that were the case, that's not the right comparison. The comparison has to be of the combination of extradition to Sweden plus rendition from Sweden vs. the easier of extradition or rendition from the UK. Since, after all, he was already in the UK, not in some indeterminate state that the US could collapse to either Sweden or the UK at its preference with no effort.
Extradition and rendition aren't equivalent though.
In the rendition case, you avoid the possibility of airing all sorts of dirty laundry in a foreign court, so it makes sense that they'd pursue that for as long as they could, and once that was not an option pursue extradition while heavily limiting public access (as they have here).
Even if that were the case, that's not the right comparison. The comparison has to be of the combination of extradition to Sweden plus rendition from Sweden vs. the easier of extradition or rendition from the UK. Since, after all, he was already in the UK, not in some indeterminate state that the US could collapse to either Sweden or the UK at its preference with no effort.