Hstore VS. JSON

Hi,

I'm in the process of implementing a table for storing some raw data in the format of a hash containing one level of
keysand values. The hash can be quite big (up to 50 keys pointing at values varying from one to several hundred
characters)

Now, I'm in doubt whether to use JSON or Hstore for this task. Here is the facts:

- I'm not going to search a lot (if any) in the data stored in the column, i'm only going to load it out.
- The data is going to be heavily updated (not only inserted). Keys and values are going to be added/overwritten quite
sometimes. 
- My database's biggest current issue is updates, so i don't want that to be a bottle neck.
- I'm on postgresql 9.2

So, question is: Which will be better performance wise, especially for updates? Does the same issues with updates on
theMVCC structure apply to updates in Hstore? What is taking up most space on the HDD?