Which method is regarded as best practice?

Cast first?

public string Describe(ICola cola)
{
    var coke = cola as CocaCola;
    if (coke != null)
    {
        string result;
        // some unique coca-cola only code here.
        return result;
    }
    var pepsi = cola as Pepsi;
    if (pepsi != null)
    {
        string result;
        // some unique pepsi only code here.
        return result;
    }
}

Or should I check first, cast later?

public string Describe(ICola cola)
{
    if (cola is CocaCola)
    {
        coke = (CocaCola) cola;
        string result;
        // some unique coca-cola only code here.
        return result;
    }
    if (cola is Pepsi)
    {
        pepsi = (Pepsi) cola;
        string result;
        // some unique pepsi only code here.
        return result;
    }
}

Can you see any other way to do this?