struc defines a new macro with all the normal features of a macro, except that it requires a label first, but otherwise there is no difference. You can make it do almost anything you need. If you have special or unusual requirements then this might be the one you want.
struct is already a macro with a whole lot of extra processing to define things like the size of each element, and it allows union and various other things. But the usage is mostly fixed. You can't make it do special processing for example, but it can already do many things so it might be just what you need for a particular purpose.
To decide which one to use it depends upon what you need from it. It would be too complex to try to enumerate all the various situations that could come up. Your question is very broad. If you have a particular use case then please ask about that and we can try to help you.
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