Correct use of C# properties

private List<Date> _dates;

public List<Date> Dates
{
    get { return _dates; }
    set { _dates = value; }
}

OR

public List<Date> Dates
{
    get;        
    set;    
}

I have always used the former, is that incorrect or bad practice? It never occurred to me that I could just use the second option. I do like having my encapsulated variables to begin with an underscore so I can distinguish them from method parameters. And I've just always done it that way.

Is it possible that using the first option would result in an extra List<Date> object being instantiated and then the whole of _dates being replaced with value, or is it more intelligent than that?

Also, which is the most prominent in industry or is it totally subjective?