The choice between ranger and assassin depends on whether you want to focus on ranged or melee.
If you want to do most of your damage from a distance, to drill the mob full of holes and never take a hit, ranger is your class. If you wanna step in close and watch the light go out of the mobs eyes as you drive the point of your blade home, assassin is for you.
Both of these classes can shed hate with a variety of combat arts. Each is considered a tier one DPSer, (although assassins do more dmg than rangers as a rule). Both make excellent soloers, are welcome in both small & full groups and bring something unique to any raid. Either is worth trying out... your call!