The determining factor as to wheter an aircraft has a 'conventional' tail or 'T' tail is how 'clean' the airflow over the horizontal stabilizer is. Conventional airliners with wings mounted to the bottom of the fuselage and engines in pods under the wing (B737, A320, etc.) have relatively clean airflow at the tail, so they have a 'convetional tail.'
Aircraft with tail-mounted engines (B717,B727, etc.) have engines where the horizontal stabilizers would normally be placed, so they are placed at the top of the fin.
Aircraft with wings mounted to the top of the fuselage and engines mounted in pods under them (BAe 146), don't have 'clean' airflow at the tail because of the wings, so the horizontal stabilizer is placed at the top of the fin.