I agree with willful wizard, he's got a good answer, but I'd like to add my two bits here too:
It can depend somewhat on playstyle. Each of the different races has the ability to react better using different options:
Zerg - Unit response:
With their larvae, you will probably have extra larvae and if you need extra units, you can get them quicker, because in midgame, you might have 10 larvae sitting around waiting to be morphed into something.
Protoss - Building response:
With the fact that a single probe can queue up 10 buildings as quick as he can move to new sites, this gives the Protoss the ability to respond to building needs quicker than other races. Probes don't get consumed, or even work for the creation of a building.
Terran - Ability Response:
Terrans seem to have more abilities with their units, and have a greater flexibility with what the units can do. Tanks can be in siege mode or in tank mode, vikings can be in the air and the ground. Healing, jumping, attacking buildings, attacking air (as well as ground) are all things that their tier 1 - 1.5 units can do.
So, maybe you find being able to respond with your current units easier, or maybe you find being able to respond with new units easier. It depends a little on playstyle.