Usage of “millar” vs “millón”

Question

First the context. There are two similar words that cannot be confused:

1. Millar  →  Conjunto de mil unidades.  →  Set of one thousand elements.
2. Millón  →  Mil millares.  →  One million.

I've seen a couple of times the word millar used when millón should be used, I think because the former is less used since there are other ways to express its meaning, e.g.:

Millares de personas salieron a las calles
Miles de personas salieron a las calles

both meaning:

Thousands of people marched on the streets.

My question is the following:

Is it advisable to use miles instead of millares to avoid confusion, or is their difference actually understood among Spanish speaking people?

Answer 1

Native spanish speakers know the difference between a "millar" and a "millón"; it's a common word, so you shouldn't use one or another for fear of confusion (except perhaps if the chance of mishearing is high: in a factory with loud noise or when speaking through a mobile phone with poor reception). No problem at all in a written medium.

About what you should use, theoretically both could be used interchangeably. I would say, and please notice that this is a subjective opinion, that "millares" is a more learned, even a bit snobbish word; I wouldn't use it in everyday language, except if I wanted to give emphasis to the idea of a big number.

I'd say "millares" is more common in written media, but again, you could use both mostly interchangeably.

Answer 2

I think both mean the same: several thousands.

I think miles is more used because it is shorter and it cannot be confused with millones.