I'd pick Specter over Fiend because Specter is a recurring source of discard, whereas Fiend nails one card and that's it, no further pressure unless you play more discard. Specter's discard is also random, which can sometimes give combo and control players fits. Specter also has evasion and is a 2/2. There have been times where I've stalled the ground up with some Shades, but needed something to push through damage. That's where Specter comes in. While 2/2's for 3 are pretty slow when they're not Ritualed out first turn, Specter is the best creature with a power/ability/cost ratio. Fiend is only a 1/1, and while nabbing 1 card you want for one less B is cute, they need only bounce it or easily deal with it to get the card back, whereas the damage Specter has done to their hand will remain even after it has been dealt with.

I value Specter higher than Fiend, and I would choose him over Fiend every time.