Personally, it took me something lile 10 attempts to do the perfect K.O (but I'm a SSFIV player so I know Ryu's moves, especially when he'll launch the ultra and how to avoid it).
Instead, the 40s one took me more time (often lose because of one-two seconds missing), something like 20-30 attempts.
So I would say the timed one is far harder than the perfect K.O which give you plenty of time, two "bigs" combos and Ryu loose 90% of his health.
Edit : I finished Akuma's ones.
The 40s is more/less the same difficulty than Ryu's one, but the "Perfect K.O." is far harder, it took me a little more than one hour (Jump and throw a fireball, if Akuma is hit by the fireball, you can safely do your combo. He never used his focus when I started a combo with an air fireball).
The 10 wins in a row aren't really hard, I lost at 8 wins the first time (and not even because of a KO but becase of the time limit), and managed to do it the second time I tried (Akuma seems pretty weak in this mission compared to the "Perfect K.O." one). When Akuma choose his first ultra (every two rounds) and when the jauge is filled, back jump and throw some fireballs because he'll likely use it and this method (the fireballs) break the ultra if you back jump when throwing them.
Just need to finish the download of the PART IV then finish it to see the true ending and doing the last achievements after, then this game will be 100% finished for me