Better shooting, better passing, equal if not better rebounding, better bench, higher point differential, slightly more athletic from 1-8, slightly younger, better matchups.
The Spurs are better defensively, but the gap isn't that wide, considering that the Suns have done a great job defensively against the Spurs this year. The Spurs are strengthened by the best coach in the NBA, and the smartest, most unstoppable power forward in the league. This is why they execute better in the clutch.
If the Suns had a better coach, we'd have won last year, and well win this year, too.