Present vs. Present Progressive Use in Every Day Speaking

[Awaken]

What is the most common way to say the following?

"I am writing a letter."
Estoy escribiendo una carta. o Escribo una carta.?

"John is waiting in the car."
Juan está esperando en el coche. o Juan espera en el coche.

In the books I am reading, the translation for the present tense in Spanish always includes the English equivalent of present progressive. For example:
Yo hablo = I talk/I am talking.
Él escribe = He writes/He is writing.

What is more common? Are both considered perfectly acceptable?

[AngelicaDeAlquezar]

In Mexico both ways are perfectly acceptable (and mean the same) in all social contexts, although present progressive tends to be a bit less formal.

[wafflestomp]

What is the most common way to say the following?

"I am writing a letter."
Estoy escribiendo una carta. o Escribo una carta.?

"John is waiting in the car."
Juan está esperando en el coche. o Juan espera en el coche.

In the books I am reading, the translation for the present tense in Spanish always includes the English equivalent of present progressive. For example:
Yo hablo = I talk/I am talking.
Él escribe = He writes/He is writing.

What is more common? Are both considered perfectly acceptable?

Both are acceptable in most situations. The present progressive is a little different in Spanish than in English. If I want to tell my friend who asked what I was doing I wouldn't say "I buy shoes at the store" in English we'd say "I am buying shoes at the store". The first sounds very strange. However, in Spanish, both sound just fine.

There are some different rules, such as the Spanish present progressive generally being for the exact moment. If you want to say, for example, my mother is coming at 4:45, which is the normal way in English, you wouldn't say "Mi madre está viniendo a las cinco menos cuarto" you would most likely just use the simple present or the simple future. "Mi madre viene a las cinco menos cuarto" or "Mi madre vendrá a las cinco menos cuarto".
I got into this discussion with my aunt from Spain about the Present Progressive and it's differences from how we use it in English.

[CrOtALiTo]

What is the most common way to say the following?

"I am writing a letter."
Estoy escribiendo una carta. o Escribo una carta.?

"John is waiting in the car."
Juan está esperando en el coche. o Juan espera en el coche.

In the books I am reading, the translation for the present tense in Spanish always includes the English equivalent of present progressive. For example:
Yo hablo = I talk/I am talking.
Él escribe = He writes/He is writing.

What is more common? Are both considered perfectly acceptable?

As Angelica's said, her answer are correct.
You should both choices.;)

[Awaken]

Thanks for the answers everyone!

Wafflestomp, thanks for those example sentences as well.

[CrOtALiTo]

Was a place help you.

You're welcome.
Don't hesitate to ask us again.

[JPablo]

Thanks for the answers everyone!

Wafflestomp, thanks for those example sentences as well.

I agree with the answers above, but I can tell you that if you have any question or doubt in terms of using one or the other, go for the present form and you'll be right more often than not. (Specially in Spain.)

You'd never say,
"España *está jugando la final contra Holanda el domingo" :bad:
Rather,
"España juega la final contra Holanda el domingo" :good:
or,
"España jugará la final contra Holanda el domingo que viene" :good:
During the time of the match, however, when the action is actually taking place you can say,

"España está jugando contra Holanda en estos momentos con el juego fluido y dinámico que la caracteriza... en estos momentos Xavi le pasa Iniesta, Iniesta regatea a dos defensas, haciendo un pase magistral a Villa que sale disparado hacia el guardameta holandés disparando a bocajarroooo yyyyy GOOOOL-GOL-GOL de Villa... ¡señores! ¡Villa, Villa, MARAVIALLA!" :D :D

(Sorry, I got carried away... I don't give you the translation 'a priori', but I can translate it for you or answer any question on my last example, ;) if you don't get all the 'nomenclature'...) :)

[chileno]

Now that I think about it, after reading JPablo and the other replies:

In Chile we use it also for the future, just what JPablo says Spaniards wouldn't ever do it.

Estoy yendo al cine el sábado por la noche. - I am going to the movies Satuday night.

If someone asks me What are you doing? (Two translations in Spanish and their corresponding answers, in general we wouldn't mix the answers but it can be done, because they mean the same as Angélica exposed already)

¿Qué haces? - (Yo) Compro zapatos.

¿Qué estás haciendo? - (Yo estoy) Comprando zapatos.

[CrOtALiTo]

Yes Chileno.

You are right.

Porque lo haces this word make me think in the future.
Why do you do it?

A donde vas.
Where do you go?

I think what the feature is in the mostly of the phrases said in Spanish, as you can see is common the use of the future in the long and short sentences.

What do you think about?