There is no difference, the former uses the current page instance and it‘s Cache property, the latter uses the static approach via HttpContext.Current.Cache which would work also in a static method without page instance.

Both are referring to the same application cache.

So you can get the Cache via Page, for example in Page_Load:

protected void Page_load(Object sender, EventArgs e)
{
    System.Web.Caching.Cache cache = this.Cache;
}

or in a static method (which is used in a HttpContext) via HttpContext.Current:

static void Foo()
{
    var context = HttpContext.Current;
    if (context != null)
    {
        System.Web.Caching.Cache cache = context.Cache;
    }
}

I find following detail from http://theengineroom.provoke.co.nz/archive/2007/04/27/caching-using-httpruntime-cache.aspx

For caching I looked into using HttpContext.Current.Cache but after reading other blogs I found that caching using HttpContext uses HttpRuntime.Cache to do the actual caching. The advantage of using HttpRuntime directly is that it is always available, for example, in Console applications and in Unit tests.

Using HttpRuntime.Cache is simple. Objects can be stored in the cache and are indexed by a string. Along with a key and the object to cache the other important parameter is the expiry time. This parameter sets the time before the object is dropped from the cache.

Here is good link for you.

Another good resource.