I prefer IAS hold. I've seen too many pilots crash in the sim because they had ALT engaged and forgot to add enough power to stay ahead of the power curve, resulting in going below Vmin, and losing control. There should be two pilots watching for MDA, so that should never be a problem. Letting the FD control the airspeed makes it a little easier to control everything else. At MDA, you just increase collective to maintain altitude, and the airspeed stays the same, minimizing necessary control inputs. I only use ALT hold in cruise, and not always then. In heavy turbulence, it will put the aircraft in unusual pitch attitudes, and makes the ride very uncomfortable. If there is turbulence, I disengage the altitude hold and let the aircraft float up and down a hundred feet or so.