I prefer the send form.

foobar.rb

# Not clear in the rdocs what the behavior will be when you pass nil!
def foo(val = nil)
  val ||= "default"
end

# This one provides much better documentation to the reader, especially "rdocs"
# (At least, I *think* rdocs show the default values...)
def bar(val = "default")
  val
end

puts foo
# "default"
puts foo("foo")
# "foo"
puts foo(nil)
# "default"

puts bar
# "default"
puts bar("bar")
# "bar"
puts bar(nil)
# nil