Let's not be hasty, Jesse should be >>>>> Kevin Spacey's Luthor.
The origin of the two says they met when Lex looked about the same age as Clark Kent around late 20s early 30s. Both Cavill and Eisenberg are 30 years old so that part works.. Also with the way the costumes/CG was done in the first one, I'm almost positive they will be able to turn him into a legit Lex Luthor.
Eisenberg has always played an intellectual, hipster, kinda douchebag in his movies.. That kind of character genre can easily be translated into an Evil Genius, especially after Now You See Me.. Honestly after sitting on the news for a day, I think this was best for business even though Bryan Cranston would've made a NASTY Lex but they most likely had to stick to the budget and go with Eisenberg instead of Heisenberg.. And I would NOT be surprised one bit if his performance overshadows Affleck's Batman.