Your example isn't very compelling, but for longer strings with more complex formatting, I'd say .format() is pretty compelling. I find that in general
'{x} blah blah blah {y}'.format(x=x, y=y)
is more readable than
'%s blah blah blah %s' % (x, y)
even if the former is a bit longer.
On top of that, there's a whole bunch of stuff you can do with .format() that just isn't possible with %.
On top of that, there's a whole bunch of stuff you can do with .format() that just isn't possible with %.