I'd take the AI, I mean a rate one turn can be worked out with a relatively simple calculation if one didn't have the turn co-ordinator but an AI tells you everything you need to know really about what the aircraft is doing.
That said, I wouldn't do away with the turn co-ordinator/turn and slip in favour of a back-up AI, it would make more sense to keep it and add an AI somewhere else in the panel.
G1000 system of course if one loses the AHRS or the screen you are left with the back-up AI and no turn co-ordinator, which I'd much rather have than just a turn co-ordinator.