I think the key issue for assassin is if you have played Hoth through. This is important because after Hoth you will get a healer companion. IMHO, to be successful in soloing you will need a tank and a healer. If you choose a sorc then you will have Khem, who will be your tank. For an assassin, you will need to wait quite a while before you will get a healer.
IMHO, assassin´s greatest strength is the combination of stealth, force speed and mind maze skill (1 minute stun). Using those skills well will allow the assassin (or the shadow) to be the best infiltrator in the game. You can easily choose which group you want to kill and which one you want to avoid. As an assassin I around level 45, I was able to do a heroic 4 droid factory quest at Alderaan because I was able to sneak in and stun humans who coordinated the droids and then I sneaked past.