Spurs
Pistons
Both have overcome adversity and beat quality opponents. Spurs and Pistons have been tested. Miami has faced nobody and Suns faced only 1 quality opponent.
Pistons beat Sixers with Iverson and a quality experienced Indiana squad. Heat have a not so 100% Shaq who must be out of shape and rusty for not playing/practicing much in last 2 months.
Spurs beat the hottest team in NBA (Nuggets) and a depleted but talented Sonic squad who was playing lights out after game 2. I think Spurs lost some focus after destroying Sonics in games 1 and 2 and on top of that Sonics caught on fire coming back home. Spurs shouldn't lose focus and intensity against Suns in Conference Finals.
Pistons and Spurs are the 2 more experienced teams AND the 2 teams with a better inside presence. Both also have better depth! Big advantage!
Spurs clearly play better defense than Suns and Pistons play better defense than Heat. Don't most people feel defense wins championships? Another big advantage!