shield is andriod so no need to invest on it.
Xbox 360 still is better
A 7770 will perform better in games than the 360 can. 360 runs the newest games at only 30 fps and the lowest settings. And they do that because the xbox doesnt have the right hardware to play games at higher settings with higher FPS. The 360 wont play games worse or better because of different resolutions, you could play a game at 800x600 on the 360 and the framerate wont increase at all. As with a 7770 you could play all the newest games at a decent resolution and settings and if you have a decent CPU even better.
The new consoles are already said to release this year, this hasnt actually been completely confirmed from either sony or microsoft so I would take any word on that. They have said they will be showing them at major events this year.