Light Witch
Advantages - very high survivability due to Shield of Heaven and Light Shield. Just need to comp for MA and you are basically set for PvM. Best option for PvP as well. Radiant Strike way better than Gravity Crush in PvM.
Disadvantages - some players with light Witches find that SoH and Light Shield make the game too easy and/or boring because of the increased DP and the second HP bar provided by SoH (which replenishes rather fast). I am not one of them, but just echoing what others have said. Not as good for bossing as a Dark Witch or a SM with decent HP/DP are.
Dark Witch
Advantages - very few players go this route, so if you like to be unique, this would be an adequate choice. A better choice than SM in PvP thanks to higher damaging one on one spells.
Disadvantages - low survivability (Dark Barrier sucks in PvM) unless you invest in HP and/or DP, which is more difficult than just focusing in one stat. Lacks a dark AoE as good as Radiant Strike (light) or the SM AoEs (Raging Storm, Staff of Thunder, Earthquake) which should not affect you very much as your other elemental AoEs should be enough, but there are some situations where the extra AoEs would be very appreciated. Overall worse than light in PvP though still a fine choice.
Soul Master
Advantages - best option for bossing thanks to harder hitting spells and blessings. Also perform marginally better in PvM and grinding thanks to space skills and the extra AoEs. Some players (including myself) take into consideration the light blue color of the SM default dress, which is cute.
Disadvantages - low survivability unless you invest in HP and/or DP. If you do PvP than a SM is a poor option due to low survivability and rather low damaging skills - while a SM move will deal more damage than an equivalent one from a Witch indeed, assuming same MA, however, this is often not possible (for example, when having equipments comped for HP, decreasing your amount of MA and/or LK) and because of low survivability (which is nowhere close as one of a light Witch). They perform worse than Dark Witches in PvP because the latter have higher damaging one on one spells.