Blink Dagger and Force Staff are regarded by many as very similar. Some even go so far as to say Force Staff is the poor man's Blink Dagger.
However, each has its own uses. Heck, a lot of players pick up both on certain heroes. Anyway, here's the breakdown (note: bolded is better option):
|Category||Blink Dagger||Force Staff|
|Usage||Self||Any Unit (800 Range)|
|Parts||1||2 + Recipe|
|Stats||N/A||+10 INT, +3 HP Regen|
|Cooldown||14 sec.||20 sec.|
|Distance Traveled||1,200 (4/5 max if target outside range)||600|
|Travel Time||Instant||0.3 sec.|
|Disabled||3 sec. after attacked by Player Owned Unit (or Roshan)||N/A|
|Magic Immunity||Can use while BKB is active||Cannot use on Magic Immune units|
|Shows Direction of Travel||No||Yes|
Destroying trees is the only neutral / situational piece here. Destroying trees is good for anti-juking and destroying Nature's Prophet's Sprout. However, with Blink Dagger, you can blink into the trees and teleport to safety.
As we can see, both have pieces that are good and bad. They're situational pick ups.
One last tid bit is that Blink Dagger is not allowed on Pudge or Vengeful Spirit. This confuses a lot of players because it was thought to be an engine limitation in DotA that should be gone in DotA 2. The only real reason this exists is anti-trolling. You can easily put people in bad positions by using blink and the Hook/Swap.