How to specify subnet with no preference like web interface in a template?

While creating a spot instance using console, it is not compulsory to choose a subnet. I can select ...

Subnet: No preference (default subnet in any Availability Zone)

But while creating a spot instance using cloudformation template, I need to choose a single subnet and I am not sure which one to choose for best price. Does it mean that using web interface is better than cloudformation template (for spot instances)?