Sangalaki has other big fish, but I did not notice Tunas. The big stuff typically will appear and literally just hang around the cleaning stations. Big bat fish, surgeons, turtles, snappers are quite common. The odd shark now and again.
Layang Layang has better visibility and a lot of big stuff too. BUT the 50 min dives really bug me.
I think Layang Layang has better coral growth than Sangalaki, but Sangalaki growth pattern is prettier. Large white sand and suddenly a huge clump of coral, great for begineers since you can see the bottom. Layang Layang are mainly wall dives unless you are inside the lagoon.
Layang Layang has to be reached in 2 days, because the connecting flight leaves around 7:30 am. Flight, hotel, flight.
Sangalaki can be reached in one day from Singapore, but you must catch the morning SilkAir flight that can connect to the 11 am flight to Berau. If you go via Kota Kinabalu, you probably will have to stay one night in Tarakan.
I think Sangalaki is better for beginers. BUT big fish I guess Layang Layang would be better.
Besides Sangalaki, they take you out to Kakaban where the Jelly Fish are. Very very nice. Huge schoold of Barracudas and the odd shark. By special request you can also go to Maratua. There you get really powerful tidal flows and super big stuff. Ideally dive the channel close to sunset and on rising tide, you can see the big animals in action.
So when you combine all the different off sites, Sangalaki is more fun than Layang Layang. But Sangalaki by itself cannot compete with Layang Layang in terms of big stuff. In terms of small stuff, Sangalaki is much much richer.
Hope this helps.