SM Naruto vs. Pain Was a Fair Fight
I believe that Naruto vs. Pain was completely even fight. Why I think this boils down to one major thing: They both had some factor aiding them in the fight.
For Naruto: The list below:
* The Deva Path had to recharge his Shinra Tenshi/Bansho Tennin for 5 minutes.
* Naruto had 3 SM clones waiting to be summoned from Myoboku.
* Naruto had knowledge of Pain's powers thanks to Katsuyu.
* (Later in the fight) Hinata jumping in and getting severly injured invoked the Kyuubi inside Naruto, breaking him from Pain's chakra rods.
For Pain: The list below:
* Pain is stronger than SM Naruto.
* Pain had some knowledge of Sage Mode from his fight with Jaraiya.
So in a nutshell, Pain had the bonus of being stronger and Naruto had the bonus of having knowledge/a plan. Those two even each other out, making it a fair fight.
Also, just to note, there was no Plot-No-Jutsu in the fight at all. Some may say "Well FELCommentary, what about when the Deva Path couldn't move 3 feet to the left to dodge Naruto's Rasengan?" Here's how I see it. At that point in the fight, what was happening could have been an endless loop: Naruto using Kage-Bushin/Rasengan and Pain repelling them with Shinra Tenshi. Kishi had to end the fight somehow, so it wasn't really his fault :shrug:
Anyway, that's why I think Naruto vs. Pain is fair.