Creatures five feet away have one-half concealment. Rogues can not sneak attack while striking a creature with concealment. Thus, creatures within an obscuring mist spell can not be sneak attacked.
Blink is a great spell. It provides you with a chance to get sneak attacks on all of your attacks, except for the 20% of the hits that don't hit because of you being blinking. Not something you should ever be light on as a DM. Don't forget the movement rate penalties either. Hmm, differences between blink and displacement? Yeah, there's plenty. Displacement skips some of the bonuses of blink while also skipping some of the penalties. Not to mention the Touch range instead of Personal.