My 2 cents on this: I like the symbols better. Just one personal experience with this is, on Mongoid, we used to accept a class on the field ,eg: `field :name, type: String`, and we always have issues around people passing classes that wont work, and also when we wanted to change the class, now we need to ask to everyone to update their code to use the new class type.
If you control the underline class looking up by symbols, you give the user less freedom, but you can make the right choices for him/her.