Exclusive units or CYL heroes to summon?

Corrin imo

Though you could wait until cyl is announced...it's not like that banner is going anywhere soon.

Not trying to make you feel down but I don't think 40 Orbs will be enough for it unless you have the % boosted to 4.5% or something already.
Personally, I would keep saving until they announce the next banner, by then you will have maybe 70 or more Orbs which pretty much guarantees you a 5* unit of your choosing.

You might not get what you want if you go for Corrin, speaking as someone who wasted 90 F2P orbs (I'm crying on the inside) on the LnD banner, and got not a single five star (not talking about focus, not a single five star at ALL)