Vampires:

There are 3 clans:

Quarra-Strength-base is Drusshetti(SW of Ulikashiku Camp)
Berne-Stealth- base is Glam Deous(West of Uvriths Grave)
Aundae-Magic-base is Alshlemech(on an island NE of Valenvaryon)

IMO, Aundae is best because of the rewards (Darksun Shield and Light of Day (mace))

Lycanthropy:

Many people have asked which path they should take; I think that it depends on rewards:

Cured

1st mission- Call Wolf spell
2nd mission- Call Bear spell
3rd mission- Stalhrim Longsword of Flame
Final mission- Your choice(Spear of the Hunter, Hunter's Amulet of Strength, or the Hunter's Amulet of Speed)

Werewolf

1st mission- Sharper Claws(150 strength)
2nd mission- Summon Bonewolf
3rd mission- Hunter's Wind(restores 1000 health, yes 1000, can be used once a day)
Final mission- Your Choice(Spear of the Hunter, Hunter's Amulet of Strength, or the Hunter's Amulet of Speed)

Guile-Spear of the Hunter
Speed-Hunter's Amulet of Speed
Strength-Hunter's Amulet of Strength

I would choose Guile so you can get the Spear of the Hunter.

Lycampires

Becoming a vampire/werewolf is possible. Just become a werewolf first, then during the day, become infected w/ vampirism. Wait till night. You are now both.
NOTE: This will glitch up your face.