If I were in the living room, I would look out the window and see the main road.
They are the same.
I find the transitive and intransitive use (if that is what it is, and it probably isn't) of "to face" is often used indiscriminately. I don't like the intransitive version with "onto" - it seems wrong.
The house faces the sea and the house faces onto the sea, the house faces towards the sea, all mean exactly the same thing.
In BE, I think the "onto" (if it is a preposition and the verb is intransitive) is rarer, and "towards" can be only be interpreted as "in the general direction of".
This might lead me to think that "onto" is also an adverb that gives the idea of the view from the house spreading out over the sea (onto is usually a preposition of motion) and prepositions of motion are rarely used as adverbs.
However, a test for an adverb is "can it be removed and the sentence stay the same?" in "the house faces onto the sea," onto can be removed with no effect on the sentence.