Signed-off-by means "I am (in part) responsible for this ending up in thid repo", i.e., you wrote part of the patch or you were on the path pushing it in.
Acked-by is used as a comment "looks fine by me" when not taking direct action yourself.
Does commiting constitute "on the path pushing it in" ?
Yes. Read the DCO if you're still unsure :-)
Ie. if I review and then commit, should I sign off or ack?