chmod a+x modifies the argument’s mode while chmod 755 sets it. Try both variants on something that has full or no permissions and you will notice the difference.
Yes – different
chmod a+x will add the exec bits to the file but will not touch other bits. For example file might be still unreadable to others and group.
chmod 755 will always make the file with perms 755 no matter what initial permissions were.
This may or may not matter for your script.