While I'm familiar with the general bent of each (Telvaani are mages, Redoran warriors, Hlaalu thieves), and I know the general recommendation is 'When in Doubt, Choose Telvaani,' because the rewards are generally considered superior, and Telvaani is the basis for some excellent mod content as well. In The Elder Scrolls III:Morrowind, several of the player joinable factions are mutually exclusive, most notably, the three Great Houses of Telvaani, Redoran, and Hlaalu.