What will happen if I use git pull --rebase ?

git pull --rebase is roughly equivalent to

git fetch
git rebase origin/master

i.e. your remote changes (C) will be applied before the local changes (D), resulting in the following tree

A -- B -- C -- D

What will happen if I use git pull --ff-only ?

It will fail.

git pull --ff-only corresponds to

git fetch
git merge --ff-only origin/master

--ff-only applies the remote changes only if they can be fast-forwarded. From the man:

Refuse to merge and exit with a non-zero status unless the current HEAD is already up-to-date or the merge can be resolved as a fast-forward

Since your local and remote branches have diverged, they cannot be resolved by a fast-forward and git pull --ff-only would fail.

\o/