Suns/Lakers is a better rivalry. There have been upsets, both teams have won series against the other, same division and there is bad blood. Plus the geographic proximity and the influx of californians into arizona.
Like Sacramento to the Lakers, until the Suns beat the Spurs in a playoff series, the Spurs will never consider you a rival, like lakerfans don't feel the Kings are anything more than an arrogant nuisance.
That said, I fully expected the Suns to win this series...I'm a bit shocked. Plus, I dislike the Spurs a lot.