Samurai and Ninja quests both take places at Shurugeon Castle which is the west side of Ishad N'ha - so it is a good idea to do them after the Serpent Temple.
I always changed my wizards to warlocks as there are more role changes available to warlock. Saz has a great list of the Role Ascensions on page 3 of the BEST TIPS thread - maybe that will help you decide.
Let us know what you decide to do!!
Wyv