I've recently had burritos from both Viva Burrito (Staniford Street) and Villa Mexico (Cambridge Street, aka the "gas station mexican").
I had chicken burritos at both to keep things even. Villa Mexico came out on top with a more distinctive, flavorful sauce. However, I still think Viva Burrito serves an excellent "traditional Americanized" burrito, with good stuff that really hits the spot. Viva B also has an excellent guacamole (did not try Villa M's guac).
Villa Mexico's salsa was WAY superior to Viva Burrito's. Viva B's salsa was good and fresh, but Villa M's salsa literally transported me back to Mexico.