By Paul Reidinger

WHEN GIVEN THE choice between a "regular" and a "super" burrito, most of us – I am speculating, but with the utmost confidence – take the super. Super is one of those great American words, like great, so that is probably a seducing factor, as is the prospect of getting something bigger, better, and fancier, all of which means, to the American ear, better. The super burrito's enhancements vary slightly from place to place, but every place I've ever been to here adds at least guacamole and sour cream, and it is the sour cream, I think, that is the magic and addictive ingredient.

Mexican sour cream is noticeably different from the gelatinous stuff we buy in tubs at the supermarket. It is a little runnier and a lot tangier, and in those respects bears at least as close a relation to crème fraîche, buttermilk, and Greek or Turkish yogurt as to yanqui sour cream – against which I have nothing, I hasten to add. But you do have to spoon it, which is a prosaic activity. Mexican sour cream, on the other hand, is usually ladled onto whatever it is you've asked it to be ladled onto, and the dripping sensuality of this casual act captures the imagination and lingers long into the act of eating the food so blessed.

The magic of El Metate is not limited to dousings of Mexican sour cream on much of the food, though those dousings don't hurt. There is the deep Mission setting too, and the white-Christmas-lights-and-rustic-wood decor, and the steady comings and goings of neighbors both Latino and Anglo, takers-out and eaters-in (there is a cozy sidewalk arrangement), many of the former being young and fitted with clips to keep their pant legs from getting snagged on the chain rings of their bicycles.

The place's reputation stretches far and wide. It was recommended to me by a neighbor, whom I promptly ran into in the take-out queue a few evenings later. He spoke reverently of the tacos, which, as it happened, I had ordered a few versions of. At $2 per, you would probably feel you were getting your money's worth even if they weren't as tasty as they are. The fish tacos, featuring breaded cod filets and shredded green cabbage, are familiar yet competitive with the best in town; the tacos al pastor, meanwhile – filled with shaved, spiced pork roasted like shawarma – carry the strong garlic-chili-powder flavor of chorizo. (Al pastor, incidentally, means, more or less, "shepherd-style" and refers to the influence of Lebanese immigrants on Mexican cuisine.)

A vegetarian burrito ($5.25) would probably have been quite good even without sour cream, for it was filled with (in addition to rice and beans, the usual suspects in meatless burritos) sticks of grilled zucchini, which had been handled lightly enough to retain some crunch yet also suffused the great cylinder with a subtle smokiness. A grilled-chicken quesadilla ($5), on the other hand, definitely depended on its jolt of sour cream to escape the doldrums. Rice and beans, I should add, are hardly slouches whether enveloped in a tortilla or not, and half pints of each ($1.25 each) would make a pretty decent cheap meal.

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Across town at La Fonda Mexican Grill, the vibe is a little more Collegeville, owing to the nearness of UCSF. The space (home last year of the short-lived Foggy Bottom Cafe) has been repainted in sunny tones of yellow, orange, and red, and the mezzanine is not the daunting inconvenience under a taqueria regime – in which you order at the counter and cart your own stuff upstairs – that it was in full-table-service days.

Much of La Fonda's menu is routine: tacos, nachos, burritos, and so forth. (The restaurant has several siblings in the city, among them Azteca Taqueria, at Church and Market, and Mexico au Parc, on South Park.) But there are also rather novel offerings, among them a torta ($5.45), which is basically a burrito on a kaiser roll instead of in a tortilla. We found the bread to be wonderfully puffy, and of course the tinga chicken was as spicy as it would have been in a tortilla, but there was significant spillage (worse than in those Carl's Jr. ads), and we appreciated anew the wisdom of the tortilla, which seals in goodness and reduces the risk of slop-over embarrassment.

La Fonda has good guacamole ($3.40, including warm, crisp chips), with a high tomato-chunk quotient, and a respectable mushroom quesadilla ($4) that takes a little longer than the other kinds to prepare, because the mushroom slices have to be griddled first, to soften them and darken their flavor. (La Fonda seems to use ordinary white button mushrooms, which tend to taste like not much without some help.) While I do not contemplate winning any awards for patience in this lifetime, I found myself not being aggravated by the slight delay, because to watch the multistage birth of a mushroom quesadilla is to be reminded that taqueria cooking is short-order cooking; the cook doesn't make it until you order it, though of course the kitchen has a large number of items simmering in pans and trays, awaiting their turn in the mix and hoping, perhaps, for a date – or at least a hook-up – with some Mexican sour cream.

El Metate. 2406 Bryant (at 22nd St.), SF. (415) 641-7209. Daily, 10 a.m.-10 p.m. Beer. MasterCard, Visa. Not noisy. Wheelchair accessible. La Fonda Mexican Grill. 712 Irving (at Eighth Ave.), SF. (415) 681-9205. Mon.-Fri., 11 a.m.-10 p.m.; Sat.-Sun., 10 a.m.-10 p.m. Beer. MasterCard, Visa. Slightly noisy. Ground floor wheelchair accessible.