The (vla-get-propertyName vla-object) form is a binding that is resolved during compilation, while the (vlax-get-property vlaobj 'propertyName) form is a binding that is resolved at runtime. The first version runs faster, but the difference in speed is frequently not noticeable.
The first form requires you to register a type library describing the ActiveX interface for your target object model with the VLisp environment before you can use it. The default Autocad object model is registered automatically. If you use vertical products (such as Land Desktop, etc.) or want to communicate with any external programs (such as Excel), you need to register the type library for that particular program explicitly as detailed
here.
This form has the advantages that it runs faster, it requires less typing, syntax-highlighting will work, and you will get a compiler error if you attempt to access an invalid property.
If you don't have the type library, or don't want to load it for some reason, you can use the second form (the vlax-get-property vlaobj 'propertyName one). The reference to the property will then be resolved while the program is running, which slows execution. Also, if you misspell a property name or try to access an invalid property, your code will compile fine, but you'll get a runtime error.