Why there are two ways of declaring variables in Go, what’s the difference and which to use?

The Variable declarations make it clear that variables are declared. The var keyword is required, it is short and expresses what is done (at the file level everything excluding comments has to start with a keyword, e.g. package, import, const, type, var, func). Like any other block, variable declarations can be grouped like this:

var (
    count int
    sum   float64
)

You can’t do that with Short variable declarations. Also you can use Variable declarations without specifying the initial value in which case each variable will have the zero value of its type. The Short variable declaration does not allow this, you have to specify the initial value.

One of Go’s guiding design principle was to make the syntax clean. Many statements require or it is handy that they allow declaring local variables which will be only available in the body of the statement such as for, if, switch etc. To make the syntax cleaner and shorter, Short variable declaration is justified in these cases and it is unambigous what they do.

for idx, value := range array {
    // Do something with index and value
}

if num := runtime.NumCPU(); num > 1 {
    fmt.Println("Multicore CPU, cores:", num)
}

Another difference: Redeclaration

Quoting from the Language specification:

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original.

This one is also handy. Suppose you want to do proper error handling, you can reuse an err variable because most likely you only need it to check if there were any errors during the last function call:

var name = "myfile.txt"

fi, err := os.Stat(name) // fi and err both first declared
if err != nil {
    log.Fatal(err)
}
fmt.Println(name, fi.Size(), "bytes")

data, err := ioutil.ReadFile(name) // data is new but err already exists
                                   // so just a new value is assigned to err
if err != nil {
    log.Fatal(err)
}

// Do something with data