I'm going with the Suns by virtue of their last victory. They really blew out the Spurs in Phoenix, and now think they can win the series.
This is not a critical game for the Spurs, but if they want to win the series, they have to win this game for psych reasons.
The Suns confidence can only go up with a win, and they know that a win will give them back home-court advantage. They will come out ready for a war, and so will SA.
Pop's has a lot of adjusting to do from game 2. The Suns were able to get out and run, and execute at a very high level, and it wasn't as though the Spurs weren't trying. IMO, the pressure is on the Spurs today, and they're not a team that will crack. But the Suns won't back down either.
Big game for both teams.