If it's a Fortune 500 company paying, then the more exotic the better. But for most consultants/engineers, the price is probably a major factor, and they aren't with a Fortune 500 company.
London is nice and central for Europe, with good transportation and easy access to flights to keep costs down.
I'm in the US, and could do London or Barcelona, etc. But not the other locations.
In the US, New York is popular, but expensive... been there, done that for conferences. Over-rated. Think outside the box a little for North America. Maybe even Toronto? Nice and cool during the summer. Would be a nice break from Austin...