From my experience, these two modes give quite a different feel on the sticks.
In ATTI mode, it seems smoother, and not as rigid, and more flowing, for smoother shots, as you can let go of the aileron/elevator stick, and the MR will slow itself down gradually (calm weather of course), and give a nice subtle change to orientation or direction, whereas in GPS ATTI, these controls are noticeably more stiff and rigid, and require bigger and longer stick inputs to get the same effect. The same can be achieved in both modes, but you have to know the bigger/longer inputs to give, and smoother flying is definitely easier in ATTI. It also seems to pick up speed quicker in ATTI, by keeping the same stick input, as opposed to GPS ATTI, and of course it stops quicker too when you let go.
I tend to only use GPS ATTI, when filming in heavy wind, so it doesn't get carried away, or for doing panning shots while moving in a particular direction - IOC 'Home Lock', or for stills.
Once you have reliable GPS HOLD, it's very comforting while filming in heavy winds, knowing that your MR is going to do 'exactly what it says on the tin'. You don't really have to compensate for wind, just concentrate on constant stick inputs with more deflection.
Hope this explains what my experiences have been...
Mav.