In my experience:
Xbox 360: Looks great when the connection is good. Whenever the connection speed changes, the picture quality changes. On a good day, it's best. On a bad day, it looks like youtube. It's like Forest Gump's life. You never know what you gonna get. On the plus side, it seems to automatically optimize itself based on connection speed. Bottom line: It ranges from being best to being the worst.
PS3: Never once had an interruption based on connection speed. It always looks good. Loads faster than 360, too. On a good day, the 360 looks better but those good days are few and far between.
Wii: Again, it loads lightening fast. No breaks in the action due to connection speed, either.Has the worst picture quality but sometime the 360 actually looks worse! Awesome interface due to the Wii remote, though.
For the record, PS3 and 360 are using wired connections and are connected to 1080p LED HDTV with HDMI Cables. Wii is Wi-Fi connected and connected to same 1080p LED HDTV with Component Cables.
