That's a tough one. Curious how others will feel about it.
Spears indicates them as synonims in figurative cases such as:
-- If you think this is wrong, you must speak up and say so.
-- I'm too shy to speak up.
Also:
---------
speak out
intransitive verb
1 : to speak loud enough to be heard <asked him to speak out or sit down>
2 :
to speak boldly or unreservedly
<stand up and speak out for the president's whole program --
Sinclair Weeks> <spoke out ... forthrightly against the carpetbag
militia -- American Guide Series: Arkansas>
3 :
to express an opinion freely and frankly <everyone of us has the obligation to speak out, to exchange ideas -- Wendell Willkie>
transitive verb : to make known verbally : DECLARE <spoke out his
mind and showed that he was not too well pleased -- Augustus Jessopp>
speak up
Function: intransitive verb
1 :
to speak strongly or vigorously -- usually used with for <speak up for truth and justice -- Clive Bell>
2 : to speak loudly and distinctly <was told to speak up as the people ... could not hear him -- B.L.K.Henderson>
3 :
to express an opinion freely and fearlessly <we'll never find out how we feel about one another if we don't speak up -- D.B.Chidsey>
http://unabridged.merriam-webster.com
----------