Based on what I was able to find, I think they are able to summon different types of toads based on their (the animal's) skill level or the matter at hand that needs attending to.
The ninja normally signs a blood contract with the animal species,
which allows them to summon different size and skill levels of
creature. The ability to summon more advanced animals largely depends
upon the amount of chakra the ninja can muster to perform the jutsu.
By signing this contract, the creatures themselves can also reverse
summon the contract signer to their location. (src)
Additionally, as per the wiki on Summoning Technique.
While most summoners have only been shown capable of summoning one type of creature, Nagato (through the Animal Path) was able to summon several different animals and even people, and seemingly did not require a blood offering when performing this technique. Sasuke is able to summon both snakes and hawks, though he has never been seen summoning both species at the same time.
Another determining factor on which toad is summoned might be moliding chakra poorly (but that was long ago) and rather unlikely to happen again.
In conclusion, a plausible answer would be that they are able to summon any animal of the species they have signed a contract with depending on their current need and or the situation, or the amount of chakra they have at the time. For the former, it would most definitely be by choice that Naruto would summon Gamabunta rather than Gerotora in a battle.