difference between "${1+$@}" and ${1+"$@"}

setu.bash

#!/bin/bash
set -u

argtest() {
    printf '$@: '
    echo "$@"
    printf '$# '
    echo $#
    printf '$1 '
    echo $1 ;shift
    printf '$2 '
    echo $1 ;shift
    printf '$3 '
    echo $1 ;shift
}

echo 'case1: ${1+$@}'
argtest "${1+$@}"

echo 'case2: ${1+"$@"}'
argtest ${1+"$@"}

# --------------------
# Result -> no difference?
# --------------------
#     $ bash --version
#     GNU bash, version 4.3.42(1)-release (x86_64-apple-darwin14.5.0)
#     Copyright (C) 2013 Free Software Foundation, Inc.
#     License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
#     
#     This is free software; you are free to change and redistribute it.
#     There is NO WARRANTY, to the extent permitted by law.

#     $ ./setu.sh "a a" "b b b" "c"
#     case1: ${1+$@}
#     $@: a a b b b c
#     $# 3
#     $1 a a
#     $2 b b b
#     $3 c
#     case2: ${1+"$@"}
#     $@: a a b b b c
#     $# 3
#     $1 a a
#     $2 b b b
#     $3 c


#     $ ./setu.sh
#     case1: ${1+$@}
#     $@:
#     $# 1
#     $1
#     $2 ./setu.sh: line 12: $1: unbound variable