In very simple words
MicrosoftDynamicsAX_model
Collection of objects is called Model. That was what introduced since arrival of AX2012; i.e. your objects(forms, classes etc. are saved in Database as models). All you customization ect. is also saved in this Database.
MicrosoftDynamicsAXBaseline
This is used just for upgardation purpose such as if you are going from AX2009 to AX2012, or from AX2012R2 to AX2012R3.