The two are completely different, and can not be compared.
./ means the current directory.
. ./ means nothing like as it is, running will print the following error:
-bash: .: ./: is a directory
However, it needs to be broken down in two:
. command is a shell built-in which is a synonym to the
source command. It takes a file and not a directory as a parameter which is why you would get the error above.
A very command example that you might have seen is the following command:
$ . ~/.bashrc
And that is exactly synonym to
$ source ~/.bashrc
You can find their manual by running
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell environment and return
the exit status of the last command executed from filename. If filename does not
contain a slash, filenames in PATH are used to find the directory containing
filename. The file searched for in PATH need not be executable. When bash is not
in posix mode, the current directory is searched if no file is found in PATH. If
the sourcepath option to the shopt builtin command is turned off, the PATH is not
searched. If any arguments are supplied, they become the positional parameters
when filename is executed. Otherwise the positional parameters are unchanged. The
return status is the status of the last command exited within the script (0 if no
commands are executed), and false if filename is not found or cannot be read.