How to best implement a data model

UserListModal.m

// Expose the internal data store. (Easiest but seems error prone)
@interface UserListModalExposeData : NSObject {
    NSMutableArray *userList;
}

@property (nonatomic, retain) NSMutableArray *userList;

- (void) someAdditionalMethod;

@end


// Attempt to encapsulate NSArray. (Good but a hassle and seems redundent)
@interface UserListModalEncapsulate : NSObject {
    NSMutableArray *userList;
}

- (id) init;
- (id) initWithContents:(NSArray *)data;
- (id) setData:(NSArray *) data;
- (void) addObject:(id)object;
- (id) removeObjectAtIndex:(int)index;
- (id) objectAtIndex:(int)index;
- (int) count;

- (void) someAdditionalMethod;

@end


// Attempt to subclass - Will Fail. NSArray is a cluster. Subclassing is bad (read: confusing)
@interface UserListModalSubClass : NSMutableArray {
}

- (void) someAdditionalMethod;

@end