They update in real time, but Waze predicts traffic, which is better. For example, in the Bay Area, if you are coming from Sunnyvale and going to Berkeley, you could go up the peninsula side or the east side. GMaps might tell you to take the peninsula route, because right now, the bridge is clear. Waze however will predict that the bridge will be crowded in 40 minutes when you get there, so it will take you up the east bay side.

GMaps may notice the bridge is crowded when you get there, but at the point it can't reroute you, because you're already on the wrong side of the bay, so it will just update your ETA.

What I've found is that the Waze ETA is almost always accurate when I start the trip, whereas the GMaps one keeps updating as I go.