You have seen how =. and =: are different when used in a verb definition.
When you execute sentences in the ijx window you are not executing them inside a verb so the =. and =: have the same effect. In the ijx window:
a =. 123 a 123 a =: 234 a 234
In the ijx window the =. and =: copulas are the same and it doesn't matter which you use to define a name as they both define the global name. The =. is easier to type and tends to be the one that is used. In a strict sense it would be better to explicitly use =: when defining a global name.