Salut, quick question, I know si and tellement can both mean "so" as a matter of emphasizing how much, but I'm not clear which to use in the context I'm thinking of.

If I'm saying someone is "so cute", or "so smart", which would I use? Elle est "si intelligente"? "tellement intelligente"? Do both work? Neither? Is it one of those cases where u just wouldn't phrase it like that in the first place?