
There is no difference. Both are representations of requirements, including other information that may be useful to understand requirements.
This view is not generally accepted. Some of my colleagues in requirements engineering research see the following difference. If the representation is with diagrams, where text is in natural language, then it is a requirements model. If the representation is in a mathematical logic, then it is a requirements specification.
They are right that it is not the same to represent requirements with diagrams and as formulas of mathematical logic. Syntax is different, rules for interpretation and computation are different, and so, your reading will be different.
I see no benefits in basing the difference in the properties of the language used for representation. Although diagrams can be unrelated to a mathematical logic, you can have those which can be fully rewritten in one. The Techne language is one such example [1]. When the language is such, that diagram/logic difference falls apart, and the model and specification are the same thing.
References
 Jureta, Ivan J., et al. “Techne: Towards a new generation of requirements modeling languages with goals, preferences, and inconsistency handling.”Â 2010 18th IEEE International Requirements Engineering Conference. IEEE, 2010.