Honestly, look at it like this. Corvus is still a great champion (even though I don't like the champion at all) and is still useful in plenty of content. R5 Corvus allows him to finish fights much faster, leaves more room for error, and less risk against high health pools. A R4 Corvus just doesn't measure up to a R5 because of this. You're not going to be doing 6.4 with him or anything but he can destroy AQ, AW, Act 5, etc. If you love him as a champ, rank him up!
To the dude who said he used Corvus for act 6 like CMM couldn't, she got me through the first 5 or 6 fights of 6.2.6 without losing almost any health so there's that...
Comments
As for who to rank up between her and Corvus, it depends. Are they duped? What signature level?
Can’t go wrong with either Corvus or her if that’s what MM stands for.