Probably not; snow is pretty much the same everywhere; though the view is better from a helicopter IMO.
My recommendation would be Coach in, Fly out. The drive into Milford is spectacular.
I agree with post #1 the coach will stop at all the touristy spots on the way in and drives straight back so I would take the coach in enjoy the journey and attractions along the way and fly back.
PS like Mt Cook the flights are weather dependant too.
For me, a heli experience is unbeatable, so if you had the budget for it, then go for it.
id definitely drive in and heli out. for me the drive into milford is the most spectcular part. Mirror lakes, the entrance to home tunnel, going through homer tunnet, out the other side and wow. then chasm creek. I wouldnt want to miss those.