With heading to the airport at that time of the morning any of the hotels will be fine, but the IC Grand Stamford's a little closer due to not needing to cross the harbour first on the way to the airport. Traffic's not going to be an issue then if you go by taxi or if you choose the Airport Express Train it commences at 5:50am and would still get you to the airport with plenty of time if you chose that.

On the plus side by taking the Airport Express Train you could check-in at the station - in fact if you took the airport express from the airport you could check in for your return flight before you headed to the hotel if you really wanted if you were flying on any of the following airlines (they all allow check-in 24 hours in advance):

Aeroflot
Cathay Pacific Airways
China Airlines
Dragonair
Eva Air
Mandarin Airlines
Qantas Airways
Singapore Airlines
South African Airways
Thai Airways International PCL
Vietnam Airlines.

As for allowing early hotel check-in, when I was just an Ambassador I stayed once at the IC Grand Stamford and arrived at 8am or so and they checked me in without any problems - though that was a couple of years ago now. Can't comment on the CP Causeway Bay, perhaps others can say one way or another?