Oh man, my buddy's giant mirror ripped right out of his plaster wall with regular toggles. He had to switch to those snap toggles and it's been solid since.
But what about the hollow space behind the plaster? Toggle bolts need a deep cavity to open up, and old plaster walls can be brittle. I've had better luck with snap toggles for something really heavy. They spread the weight over a wider area and seem to hold tighter in iffy walls. A regular toggle might just crack the plaster when you tighten it down.