the only difference is the size and wheels (do you really want one on wheels) . I went for the 24L as i only need it for the basic items and only go away 4/5 days max and the price was excellent.
Depends if you are always going to have electric hookup or not. Running on 12 volt will help to flatten your battery quite quickly, having a 3 way fridge (gas too) will increase your options.
Jan