Does the US position come with 5 weeks paid vacation? When the US position gets hit with layoffs, do you still have comprehensive healthcare and unemployment coverage?
Yes, EU software Eng salaries are a lot lower than their US equivalents, but quite a lot of folks are happy having a top 5% salary in their own county, without all the stresses and risks of working in the US.
As a concrete example, a few years back it leaked that Dan Abramov's (very decent) London salary was half what his US peers earned. Didn't seem to bother the man much at all...