Sigma flash confusion

I need a flash that's got more power than the little built in unit, but since I don't do any studio work I think a Sigma will cover my needs well. I shouldn't need to do much of anything with full manual mode. From what I understand the EF-530 just has another 3 meters of range over the EF-500. But what is the difference between the EF-500 Super and the EF-500 ST, besides $50? I looked at the descriptions and specs but I can't see the difference

The 500 Super DG has more manual output settings, a built-in optical trigger and can function as a master or slave.

Don't have the ST version, but have a few DG Supers. FWIW and based on my understandings...

DG Super has but ST doesn't have:
1. FP Sync
2. Built-in optical slave
3. Manual power output from 1/1 to 1/128
4. Can act as wireless master or slave in ETTL or manual modes

Other Sigma users pls correct or add to this list.

OK, I'm not too worried about the master issue, but can you elaborate on what exactly an optical slave/trigger is? Also, is FP sync different from the sync speed?

When in optical slave mode, a DG Super can be set off by any other flash simultaneously (i.e. in sync). Need to watch out for the preflash, though. On the other hand, the FP mode will allow the shutter speed to go faster than the camera's fastest sync speed. Need to watch out the flash range and power consumption in FP mode.

FP flash is another word for high speed sync. Some people use it a lot, others never do.

The optical slave is real handy for off-camera use, but you need another flash with manual capability to trigger it.

One other thing worth noting: There is no way to use the ST model off-camera (other than a TTL cord like the Off Camera Shoe Cord 2 or 3). It won't fire with a hotshoe adapter.