1080p choppy on two different GPUs, what else could it be?
Can you confirm if hardware assisted decoding is actually being used at all?
The output from mplayer and vlc (if run in a console) would help confirm or deny this if you could add it to your original question.
Try this in ~/.mplayer/config to turn it on:
vo=vdpau,xv
vc=ffh264vdpau,ffmpeg12vdpau,ffwmv3vdpau,ffvc1vdpau