Boolean in VHDL? When does '0/1' fail?
It's a matter of taste to be honest.

In the 1993 LRM there's a note related to BOOLEAN on page 36 which states that 


  type BOOLEAN can be used to model either active high or active low logic depending on the particular conversion functions chosen to and from type BIT


The implication being that BOOLEAN should be used to described functionality and converted to a "logic-level" later on.