I have recently seen all 3 of his SWs and I was surprised how good they were. Not many fans seem to notice his movies though so I recommend them this way. 
Montero’s “Thunder Over El Paso” is his best and it is even by far the best SW of 1972 that looks like it was made in 60’s heyday.
‘Django’s here … Pay or Die’ isn’t anything to write home about, but, ‘The Two Faces of the Dollar’ is very entertaining, not least because of the ‘Baron’ himself, Gérard Herter … I haven’t seem ‘Thunder over El Paso’ yet … but I will give it a chance, and get back to you.
The very fact that an SW member has seen the merits in a film, is praise enough, indeed, for the rest of us ‘SWDB’ brothers to investigate…and agree, or disagree with what he/she likes…
It is clearly the weakest of the 3 with a very low budget and a rather routine story. I still think Montero did a fine job with what he had at disposal and I liked it. Actually all of his westerns grew with me over time. Durango with a sawed-off shotgun and revolver combo is kinda cool.
The characters in his movies are great. He used over the top almost comics like characters in both “Two Faces of the Dollar” and “Durango”. I’d say Thunder Over El Paso is a little bit more conservative in this aspect. I loved it. Hopefully you won’t be disappointed. 
I’d classify Montero as a really solid genre director. No virtuoso, but certainly a dependable craftsman. While Durango Is Coming, Pay or Die is a bit of a dung pie in my humble estimation, both Thunder Over El Paso and Two Faces of the Dollar are pretty good. If you like Montero, I suggest you that you check out his Eye of the Spider: a bit of a superficial crime yarn, but well put together and quite captivating, though I cannot put my finger on why exactly.