Using a TVS diode instead of, or in addition to a MOV for AC line protection?
It comes downto what are you trying to protect against

There are four main types of transient suppression devices

Gas Tube


Protection time: > 1us
Protection Voltage: 60 - 100V
PowerDissipation: Nil
Reliable Performance:  No
Expected Life: Limited 
Other:  Only 50-2500 surges, can short powerlines


MOV


Protection time: 10 - 20ns
Protection Voltage: > 300V
PowerDissipation: Nil
Reliable Performance:  No
Expected Life: Degrades
Other:  Fusing required. Degrades


Avalanche TVS


Protection time: 50ps
Protection Voltage: 3-400V
PowerDissipation: low
Reliable Performance:  yes
Expected Life: long
Other:  Low power dissipation. Bidirectional requires dual


Thyristor TVS


Protection time: &lt;3ns
Protection Voltage: 30-400V
PowerDissipation: Nil
Reliable Performance:  yes
Expected Life: long
Other:  High Capacitance


http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF

( http://web.archive.org/web/20051001082352/http://www.onsemi.com/pub/Collateral/HBD854-D.PDF  ) 

http://www.vishay.com/docs/88440/failurem.pdf