I'd rather say
 

Como se traduce más bien diría o preferiría decir

What I'd rather say is ...
What I meant (to say) was ...
I meant to say ...
Rather, ...

"I'd rather say" would be understood just fine, but isn't as common.

so is not an equivalent to I´d say?

I use "I'd say" to state what I would say in some theoretical context; I'm not explicitly stating a preference between two or more choices.

I use "I'd rather say" when I want to indicate which of two (or more) alternatives I would prefer to say in some context; that is, I'm stating a preference.

A la hora de traducirlo "yo diría" (I'd say) que "más bien diría" (I'd rather say) que "preferiría decir", "I'd like to say"...

Digo yo, ¿no? ;-)