I think in the events where you can choose between 6 or 18 people (or anything in between), players should be able to get the same items no matter which version they choose, but that item drop rates should be higher in the 18 man version, simply because there's more people competing for the same items. An item with a 20% drop rate when fighting the alliance version shouldn't have more than around 5-6% drop rate in a single party version. Of course, if you choose to attempt on the version that is intended for 18 people with less than 18 people, you shouldn't be penalized for that, but instead get the same high drop rate split among fewer people.