I am not wrong.
They gushed over both (for good reason) and both at one time or another were told they should win.
You are right. They never gave Fantasia preference over La Toya. Simon once called La Toya the best singer in the competition, which is VERY high praise. Simon enjoyed lumping them together. I can't think of ANY cases when La Toya received less praise than Fantasia after giving an equal or better performance.
As for Diana, I do think Simon was unfair tonight, diverting attention from the performances he liked from her with petty criticisms. Like the first song, he had problems with the lyrics, which were written for her. It was strange.
But he DID bash Fantasia's first performance, which was bad, when the other judges tried to flourish the quality. He called it a complete mess.
BTW, don't worry about the Fantasia haters. I've supported her from the beginning. I also like Diana, but I would've put Jennifer or La Toya in the final two over her, two other ladies who, like Fantasia, are talented enough to make a singing career without this show.