I can find no evidence that mantids normally prey on spiders at all, large or not, though I bet it happens from time to time. However, it would be more likely that a mantis would catch a moving spider than one in a web, and I would think that a small mantis would "think" twice about attacking a larger spider. I suspect the mantis you observed was considering using the web to get off the AC. By the way, it's "praying" mantis, not "preying" (the way they hold their forelimbs makes them look as if they're praying).