On Bing (Windows Live), on your cell phone you can put in directions and it'll guide you basically by telling you what your upcoming turn is, and once you reach that turn there's an onscreen direction indicator telling you to turn (and it'll even beep).

The only thing I can see for Google Maps is it'll give you the overall directions, but you have to manually press the button for "next turn" to even know when to turn. If you don't do this, you're basically stuck at Step 1 of the directions, it doesn't "sync" to where you are in relation to the directions (and if you hit "0" to center on your GPS location, sure it tracks you in realtime, but you still lack the guided directions).

The only reason I'm even considering using Google Maps for my GPS directions is because Bing just seems to have trouble rerouting a direction if you happen to go another way, most of the time it works, but if you miss a turn on a freeway and have to reroute, it's hit or miss weather it's smart enough to reroute you in the most intelligent way.