To answer your first question... .format just seems more sophisticated in many ways. An annoying thing about % is also how it can either take a variable or a tuple. You'd think the following would always work:
"hi there %s" % name
yet, if name happens to be (1, 2, 3), it will throw a TypeError. To guarantee that it always prints, you'd need to do
"hi there %s" % (name,) # supply the single argument as a single-item tuple
which is just ugly. .format doesn't have those issues. Also in the second example you gave, the .format example is much cleaner looking.
Why would you not use it?
- not knowing about it (me before reading this)
- having to be compatible with Python 2.5