From the pictures I have seen they seemed pretty comparable to me. Lusitania had 2, 4,6 and 8 person berthing compartments for 3rd class where most ships before her had large barracks like spaces for 3rd class. Some of the 3rd class cabins from pictures I've seen looked a little better than Titanic's.While watching one of the better real-time sinking animations of the Lusitania, I noted that there was one clip of flooding of the Third Class dining room that lasted several seconds. Later, I compared that view with actual photos of the same room.
It seemed to me that at least the Third Class dining room on the Lusitania was rather better appointed than its equivalent on the Titanic. Of course being larger, the White Star liner probably allowed more public space for its "steerage" passengers but how did they compare overall in terms of facilities?