I use both for most day to day leveling/questing, The clanfear is the 'tank' as it has much higher HP and defense then the winged twilgiht, the winged twilgiht howver has ranged magic attacks and its morphs have things like the ability to heal you when you get low on health, or increase magicka regen in an aura around it for you and allies.

The WT will die much much easier if you only use it for some stronger mobs.