Ah, yeah, I'd already heard the 'Sidious was only setting up Anakin' reasoing. I don't buy it for the following reasons:
A: I regard Sidious as a smart, effective villian. Despite the assertion from some posters that he's a 'chump' bad guy, you'll notice that Sidious is the one who actually managed to take over the galaxy. Sure, he used sneaky, underhanded means to do so, but he succeeded where the other Sith had continually failed.
To accomplish such a goal, he had to be a survivor. Survivors do not willingly place themselves into a position where random chance can end their existence. Had a good stiff wind from outside hit Mace as he stood over Sidious half-melted form, the Sith Lords reign would never have come to be. Same thing if Anakin had gotten stuck in traffic. Sure the force gives him foresight, but combat situations are fluid; even the Force would not account for all variables that might end up killing the Dark Lord if he deliberately placed himself in a helpless situation.
That's also ruling out Anakin. Sure, he made the decision to help Sidious, but given the fact that Sidious was the only Sith in the galaxy, I don't think someone as cagey as the future Emperor would be assured enough of Skywalker's decision that he was willing to put himself in front of a bullet to further tempt him when...
B: There was no real advantage in doing so. Had Sidious killed Mace, he still could've rocked Anakin's world with the phrase 'They just tried to kill me!' As instantly effective as what happened? Maybe not, but still a significant victory in the war for Anakin's soul.
Those are the 'motivational' reasons. The next two are storytelling.
C: The Emperor is more effective as a villian if Mace actually had the advantage. This is partially related to reason A, but also because it makes the final seduction of Anakin a snap judgement, the immediate and unhesitant exploitation of a situation that dropped into his lap. If I was the Emperor, I would have been expecting the final victory over Skywalker's good side to take a little longer, and I assume the guy in the movie was moving just as cautiously, which means that, though he did not expect what happened, he immediately took as much benefit from it as he could.
He does the same thing in the Senate chamber, milking his disfigurment for political capital. Playing possum for Mace kind would ruin the idea that he's so competent, as least as far as I'm concerned.
D: The Emperor learns from his mistakes. Mace was probably not as powerful as Sidious when it comes to the Force, but he held a key equalizer: His lightsaber. Sidious couldn't have been as practiced at swordplay as Windu. Even with the idea that he was a master lightsaber duelist (and he's clearly good) he's spent the last several years in a situation where it'll be difficult to find time to keep his skills sharp. Thus, when he is confronted with the bald black man, he loses the sword fight and resorts to his knowledge of the force. Unfortunately, he does so in a manner than Mace can counter with his lightsaber.
Now here comes the learning part. Yoda is acknowledged as the Jedi's greatest swordsman. When Big Evil and Little Green throw down, my general impression is that the Emperor was trying to avoid direct lightsaber action. Oh sure, they parried and riposted some, but mostly, Sidious tried to turn it into a force duel...a type of conflict he was better equipped to win, as he and Yoda were apparently a nearly-even match in that area.
Hence, he did not repeat the mistake he made with Mace, and fought Yoda to a draw.
I realize of course, that the 'reality' of the situation is entirely up to the viewer, but to me, the Emperor is a much much meaner man looking at it my way.