Side stand is preferable, as Andrew said. The bike is more stable leaning on it that than on a center stand. This is especially important in the S.F. Bay area which is prone to earth tremors/quakes.
The center stand is, however, really useful for doing chain and other bike maintenance. An alternative if you have a sport-bike is to use a paddock stand (a.k.a. race stand, a.k.a. swing-arm stand) for bike maintenance.
If using a center stand, once you’ve put the bike on the center stand, make sure you lift the side-stand up as it could cause complications when taking the bike off the center stand.
In taking the bike off the center stand, you should first be straddling the bike, then push off the stand with your feet and legs and arms (push on the bars). Don’t take it off the stand if you’re not straddling the bike to steady it.