Both are cool missions. Highly challenging and enjoyable to play.
Yuri's Final Return map : my personal difficulty rating would be 7/10, at brutal level. Nice to garrison the factory with initiates near the bridge and set up a grand cannon base. Interesting to defend your base when Yuri grows angry and exciting to observe flying yuri clones.
There is an easy way for a sudden win (you even don't need to build an offensive army), but I don't want to give a 'spoiler' to gamers who haven't tried this mission yet.
+
The Avengers map : Beginning part is pretty hard till the arrival of your mcv. After the lab destruction and heros/Einstein evacuation, I voluntarily do not prefer to kill enemy mcv's and let them build their bases, so as to enjoy a more challenging mission.
Personel difficulty rating: 9/10 (before base), 4/10 (after base).