Yes it was the first one, but Il mercenario has the advantage of being a real western, while Quien sabe is a western (despite its director denying it), but is more on the edge of the genre, is much closer to the political cinema of the 60s than the other Revolution Spags. Closer to an US forerunner like Viva Zapata (Kazan, 1954), or to Hossein’s The Taste of Violence (1961), which are both no westerns imo.
Apart from that, I prefer The Mercenary. Quiens sabe? is top 20 material, but The Mercenary is top 3.