What type of plaster should I choose for a block-carved plaster sculpture?
There are a variety of specialists which will stock a good range of plasters, these include sculpture and ceramics suppliers and well as those catering to industrial mould making. 

The plaster used for construction is not really what you want unless you find a supplier which caters for more traditional mouldings etc. 

A hard alpha plaster like CrystacalR  is very hard indeed and certainly takes some effort to carve and the various blends will give different hardnesses, setting times ect. 

I would look for a general purpose casting plaster as this should be somewhere in the middle of the range of hardness anything sold as dental plaster or plaster of paris should be fine. 

Ideally I would try to find a specialist supplier with a good range eg : http://www.specialplasters.co.uk/plasters.html

In all cases make sure you mix the plaster properly and with the correct quantity of water or you will get inconsistent hardness.