Hi,
I'm currently in the planning stages of a medium sized application.
The essence of this application will be a website, similar to hotel booking sites like booking.com
I'm deciding what data access to use...
I've narrowed it down to two options - nHibernate, or Microsoft Data Access Application Block

In what situation would either of those be a better option?

Are you hungry for an apple or an orange? They're fundamentally different. NHibernate is the way to go if you are going to do a full domain design using business objects.

DAAB is the best ADO.NET wrapper/helper library. It may be useful if you are going to extensively use datasets, hand-coded SQL, and/or stored procedures for data access that would make extensive use of ADO.NET directly.