I would say over-all that LC produces a more shallow start to great wall dives and there-by maximizes your bottom time.
These very shallow sites are the definition of Bloody Bay, with the six at the East end being some of the best-known: Mixing Bowl (aka 3 Fathom), Marilyn's Cut (aka Hole in the Wall), Donna's Delight, The Chimneys (aka Randy's Gazibo) and Great Wall East & West. The problem with this eastern half and these six mooring are crowded into a mere 860 yards of real estate, which averages out to less than 150 yards between each. That's only 1/4 of the Marine Park... and they all also have hardpan shallows; no spot reefs or spur/groove.
In the Jackson's Bight section, it initially starts at only 40-50fsw for the wall's main dropoff, but its also deeper to the sand on the inside...60 to 120...plus the wall's edge gets progressively deeper as you run west, with Cascades being quite deep (100ft). These walls are deeper than further east on the north side, as well as the walls in the Southwest corner, which both mirror the Brac. As such, the two islands average depths to the drop-off is not really as radically different as we might otherwise believe...its simply what we habituate to.
Now, if you had Nitrox the Brac can easily compete.
It does help, but it also limits your max safe depth. I find it a hard trade-off as to dive Wilderness Wall on air or 32%, as the school of Horse Eye Jacks there that school around the deepwater pinnacle frequently will go try to "hide" under the ledge on the west side at 130fsw. It can be hard to catch them out in the open:
Plus I've found that if you're chilling in the afternoons and only doing 2 dives/day, particularly with the 'regimentation' (limited bottom time allowances) of some of the dive operations, you don't really build up the N2 load to really need 32%...it tends to be more a question of how liberal/conservative one's own dive computer happens to be, and/or if you're willing to occasionally do a required 5 minute hang.
The wall on LC starts in a few places near 20ft., where on the Brac the wall (north side) starts at 60ft. which can limit your bottom time when dropping down another 30 to 40 ft., but there certainly have less dived sites at the Brac, especially my favorite Strawberry wall. There is an area you can consistently see flying gunards (Schoolbus wall), here the wall starts at near 90ft., the gunards are found at a plateau around 60ft. Then there's areas like East Chute, beautiful but you can get to 110 quickly.
And the barrel sponges there are bigger than any I've seen on LC.
Another interesting tidbit alludes to Anchor, Tunnels, Wilderness, Inside Out and Rock Monster, all on the South Side. An interesting contrast, as all of the Marine Park's walls are northside and thus, all always in shadow. Shadow can make for nice silhouette photography, but you'll never get any sunlight down into the deep along the face of the walls....a very different view.
FWIW, I've been trying to think of if there's a good equivalent to 'Lighthouse' on LC. The Brac's spur & groove frequently is of quite high profile, but I think that the "slot canyon" at Lighthouse is around 30ft of profile on a 50fsw reef.
A big part of the problem has become who will take you to the dive sites that you want to go to on the Brac.
And Little Cayman too, as there's always a diver who's never been to the island before that has his dive magazine punchlist, which is always the same few sites: 3 Fathom, Randy's Gazebo, Magic Roundabout, Marilyn's and if its a longer list, perhaps Eagle Ray or Blacktip Blvd and Great Wall West.
If you stay for 2 weeks, there will be some new divers the second week with the same exact request. Lather, rinse and repeat.
Only one dive op - the Brac Reef Resort will take you out but their choice of dive sites is questionable for a veteran, or at the old Divi - Craig who has InDepth watersports will take you out, he has good choices but charges you a pot of gold to get to the sites. This makes the Brac less desirable for someone that's been to the Brac a number of times like me and we don't like paying big bucks to get to the dive sites.
Over the past two years in particular, I've found both operations to be more willing to go to some new & different sites, such as the Prince Frederick. Plus they've been including some that are anchor drops, not moorings, such as Stingray Tower. I'll have to dig up the GPS coordinates & draw a site map; its a pretty nice one, although it can be deep if done as a second dive...although Jackson's has that same problem if you venture out through most of the swim-thru's.
-hh