Not remotely the same thing, Smaug speaks and has extended conversations and interactions with Bilbo and the dwarves so mocap work was an incredibly useful tool to, like you say, give him more personality and more realistic facial expressions. The spider version of It in the book does not speak or interact with the Losers other than acting like you'd imagine a gigantic version of a spider would (just like Shelob in LotR which is why I used her as a comparison), crawling and rearing up on it's back legs, e.t.c. The only time It speaks in this incarnation is inside their minds. Of course, they may decide to mix that up in the film, but I doubt it.
How do you make a photo-realistic (not cartoony) spider look like it's talking anyway? It doesn't have a face like a mammal or reptile (i.e. two eyes, a nose and a mouth) that would align up with a human face doing mocap work.