Force Staff VS Blink Dagger

Blink Dagger tends to be better and more flexible than Force Staff for initiation, but Force Staff is generally better for escaping since you don't get a cooldown on it by taking hero damage. It can also be easier to assemble a Force Staff since it's made of three different parts that you can gradually buy even inbetween deaths, while for the Blink Dagger you need to amass its full cost at once in order to complete the purchase. The intelligente and HP regeneration bonuses from the Force Staff can also be useful for certain heroes/situations, remember that extra intelligente provides a bigger mana pool as well as some extra damage if that's your hero's main attribute.

Blink Dagger could also be better for general mobility around the map by being able to blink around each 12 seconds as long as you don't think you'll need to use the blink aggressively until the cooldown is done, while a Force Staff can also be used on allies to manipulate their positioning advantageously, such as by saving them by creating a gap between them and a dangerous enemy who's chasing, or by pushing them towards/closer to a helpless enemy they are trying to chase, it can also be used on enemies to pull them towards your team for example.

The main difference between the two is how they are used.

Blink Dagger pros:

• longer range
• point target for precision in positioning
• superior initiation and movement
• 12s cooldown with 0 mana cost
• can single handedly turn a mediocre hero into an amazing hero

Cons

• Can only be used on self
• easily disabled
• extremely bad against certain DoT effects (Venomancer poison, Shadow strike, Radiance, etc.)
• gives no method of escape once caught out

Force staff's pros:

• easier item progression (1000 + 350 + 900 rather than a flat 2150) for supports to farm
• broader utility (can be used on self, allies, and enemies)
• counteracts specific hero abilities (Clockwerk  Power Cogs, Nature's Prophet Sprout, many slows) and improves others (Batrider Flaming Lasso, Bloodseeker Rupture )
• gives stats (better on int heroes)
• Better escape utility

Cons

• longer cooldown
• lower range
• imprecise positioning

In many cases, getting both Blink and Force Staff on a hero isn't a terrible idea as every hero can benefit to a certain degree from added mobility. However, heroes who already have good mobility (Razor, Storm Spirit, Queen of Pain, etc.) and heroes who need to prioritize survivability or damage will benefit minimally. Heroes which don't need items to be effective (most supports, Ursa, Invoker) or which require good positioning to land their spells (Windranger, Pudge, every big initiator) benefit the most from added mobility.