What is the most efficient way to store / retrieve items in asp.net httpContext.Cache

I have a website where I cache a lot of info. I am seeing conflicting information around storing stuff in the asp.net cache.

For example lets say I have this data structure:

 Dictionary<string, List<Car>> mydictionary;

I could store the whole thing with a string key as "MyDictionary" and then drill down once i pull out the object.

 HttpContext.Cache.Add("MyDictionary",  
   mydictionary, 
   null, 
   Cache.NoAbsoluteExpiration,
   new TimeSpan(10, 0, 0),
   CacheItemPriority.Normal,
   null);

 var myDict = HttpContext.Cache["MyDictionary"] as Dictionary<string, List<Car>>;

The other thing i could do is break it down and store each item in my dictionary separately in the cache (given the cache is a dictionary anyway).

 Dictionary<string, List<Car>> mydictionary;

 foreach (var item in mydictionary.Keys)
 {
      HttpContext.Cache.Add(item, mydictionary[item], null, Cache.NoAbsoluteExpiration, new TimeSpan(10, 0, 0), CacheItemPriority.Normal, null);
 }

 var myitem = "test";
 var myDict = HttpContext.Cache[myItem] as List<Car>;

Would the performance implication be very different (given i am assuming that everything is in memory anyway ?)