How do conventional and T-tails differ?
There is more to a T-tail than that:

Aerodynamics:


The placement on top of the vertical gives it more leverage,
especially with a swept tail.
Depending on wing location, it stays in undisturbed flow in a stall. Note: This is really depending on the details, the HFB-320 had a forward swept wing and a T-tail, which made a deep stall possible (and in one case fatal).
By designing the junction with the vertical well, the T-tail has less interference drag. It also helps to reduce wave drag, especially when using a well designed Küchemann body (the round, long, spiky thing on the tail junction of a Tu-154) by stretching the structure lengthwise.
It can help to increase the effectiveness of the vertical tail by keeping the   air on both sides of it separated. At the other end, the fuselage does this already, so moving the horizontal tail up does not hurt so much there. As a consequence, the tail can be built lower.


Structure:


The mass of the horizontal tail on a long lever arm (= the vertical tail) means that the torsional eigenfrequency of the fuselage will go down. This might be a problem in case of flutter.
As a consequence of the smaller vertical tail, a T-tail can be lighter. Note that the increased leverage means that the horizontal tail can be smaller as well. This reduces friction drag and is the main reason why most modern gliders have T-tails.


Control:

A T-tail produces a strong nose-down pitching moment in sideslip.

If it were not for the flutter and pitch-down, T-tails would be more widespread ...