What is more conversational - "te lo digo" or "yo quiero decírtelo"?

Question

I'm doing a Spanish grammar book, and in it I see one exercise, giving answer to questions.

The grammar topic is dative/accusative cases in same sentence. I know there are no cases actually, but my native language has cases, it helps me think this way.

Two answers given:

te lo digo _{I am telling you it / I tell you it}

yo quiero decírtelo _{I want to tell you it}

This addition of 2 endings at the end of decir is shocking me. What is more conversational of the 2 options above?

Do people use "decírtelo"? Or mostly use "te lo digo"?

Answer 1

In both te lo digo (1) and quiero decírtelo (2) there are three verb arguments: subject, direct object (the accusative pronoun) and indirect object (the dative pronoun), but the verbs and their meanings are not the same: in (1) you are saying, in (2) you want to say. But I think what's confusing you is the position of the pronouns.

The Spanish non-emphatic object pronouns (me, te, se, la, lo, le, nos, os, les) are clitic: they're normally unstressed and from the phonological point of view they're part of the main word to which they attach. They can attach to the beginning of the following word (proclitic), or to the end of the preceding word (enclitic). By orthographical convention, proclitic pronouns are written separate from the verb, while enclitics are joined to it, but that's just an arbitrary rule. (The articles el, la, los, las, un, una, unos, unas are also enclitic.)

All the following phrases are perfectly grammatical in Spanish:

quiero decírtelo
te lo quiero decir
quiérotelo decir 😦
te lo digo
dígotelo 😦

but the ones I've marked (with quiérotelo and dígotelo) are extremely unusual. You might find something like that in poetry, or in very old books, or in humoristic, affected speech, but not in real life speech today.

The reason why quiero decírtelo is natural and common is that clitic object pronouns can't be attached before an infinitive, only after. So none of these is grammatical:

*quiero te lo decir 😱
*voy a lo llamar 😱
*vas a se lo decir 😱
*fue a nos buscar 😱

The correct forms should have the pronouns either after the infinitive or before the main verb:

te lo quiero decir or quiero decírtelo
lo voy a llamar or voy a llamarlo
se lo vas a decir or vas a decírselo
nos fue a buscar or fue a buscarnos

Which one of the correct forms you use is a matter of choice. Some people prefer the first pattern, others the second.

These rules are valid for most verbal tenses and moods. But for the imperative, the object pronouns will always be attached after the verb:

dímelo ("say it to me")
dáselas ("give them[FEM] to he/she/them")
búscanos ("look for us")

Answer 2

I think the answer depends on the question, both answers are correct, but they don't mean the same.

Dime por qué estás tan enfadado.

¿Quieres que te lo diga? Pues te lo digo.

Te lo digo means I am telling you right now.

Other example:

Dime por qué está tan enfadado Luis.

Quiero decírtelo, pero le prometí no contárselo a nadie.

Quiero decírtelo or Yo quiero decírtelo means I want to tell you but usually you won't.

By the way, you can also say Quiero decírtelo y te lo digo, meaning I want to tell you, and I am telling you.