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Declare a generic variable type

I am trying to declare a generic variable type in C (I cannot use C ++), and I mean the following parameters.

Option1

typedef struct 
{
     void *value;
     ElementType_e type;
} Data_t;

Option 2

typedef struct {
    ElementType_e type;
    union {
        int a; 
        float b; 
        char c;
    } my_union;
} my_struct;

where ElementType_e is an enumeration that contains all possible types of variables (for example, int, char, unsigned int, etc.). I seem to be inclined to option 1, because I do not believe that casting will add extra computing time compared to the switch, right?

I'm just wondering which type is more useful? I know that option 1 will require use with every use / access. Are there any possible problems that may arise during casting (especially when running / compiling code on another platform, for example, 32 bits and 16 bits of micro)

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