Hi Soumendra,
As PIX said both Half and Full rate are better.
Basically Full rate means more bits uesed to represent voice and good clearity typically 13 kbps and half rate means number of bits used to represent voice is halfed using diff compression techniques and it is 5.6 kbps normally. If full rate is configured on TRX one GSM slop will carry one voice call but if half rate is configured same slot can carry 2 voice calls. So decision to use half rate or full rate is decided on trafiic requirement, no of calls taken by that TRX, Voice quality needed.