first time I raided eternity vault on my shadow, I was wearing mostly oranges and blues and even a couple greens but when it came time for the 1v1 boss fight (where everyone fights their own boss, making it a semi-viable means of comparing dpsers within the group) I finished my boss 15 or 20% ahead of a dps sage that was in mostly columi gear because for some reason they seemed to think that using disturbance at least 15 times per rotation would pull comparable dps. anyone can play a shadow, but you have to learn to watch your procs to be any good at it. half force cost on shadow strike / maul is a big deal when the full cost is half of your energy, and remembering to hit blackout for increased force regen and being opportunistic with force cloak to get an extra blackout regen in are pretty important factors also. as long as you gear pretty heavily towards crit and surge, assassins hit pretty hard.

the biggest difference between playing a shadow and playing an assassin is companion differences - shadows get a healer companion in their mid 20s and can virtually sail through the remainder of leveling without stopping much to meditate, whereas assassins dont get a healer til hoth (early 40s) so it's slightly more sensible to level a shadow as infiltration (mirror of assassin's deception tree) with a healer around while leveling an assassin would probably be easier using the madness tree (mirror of consular's balance) due to the various return heals, though dont let that impact your decision too hard as I'm sure you could level a shadow with balance and I know an assassin can be leveled as deception