Safety wise, what is the best system.
Having a kite that when released from the chicken loop remains attached to the safety leach connected to 1 line (like the Slingshot's ) our 2 lines ( like the Oceanrodeo's)?
Well first of all I think there are more important things than single line flag vs mini 5th line etc., but to answer the question
1) 5th line is safest. Even if you have snapped both center lines, the 5th line is like a backup. It also does not let the kite rotate and flap around like a single center line will do in high winds.
2) single center line like slingy is next safest. But it is not as convenient as mini-5th line because relaunch is difficult as the bar zooms at least 10m up the lines.
3) mini-5th line like ocean rodeo is next. The reason for this is that the mini-5th line requires the bridle to be operating correctly. So for instance if a pulley freezes up and you hit the QR it will most likely fail. By contrast, a single center line will flag out a kite even if your have a frozen pulley or bridle wrap. mini-5th line in general will not safely flag a kite on a bridle wrap.
Having said that there are actually three more important things for safety than the choice of safety system, they are
1) do not buy a QR that quits working if it's clogged with sand
2) leash to the front of your harness not the back
3) practice using your safety system, and make sure it is in good working order before you kite.
I think you can kite safely with most modern quick releases, each system has its advantages and drawbacks, I like single center flag partly because the V comes all the way up to the bar, where the mini-5th line has a Y, so you can't grab a single center line on the left or right, it's something I do for light wind relaunch. One thing I do like about the mini-5th line is that it will work even if you snap a center line, where with a single line flag out, if you hit the QR and snap your safety line, you just released the kite and now you have to go chase it down.
One more thing, a lot of kites designed for single center line flag out lose power if you try to fly them with a mini-5th because the Y pinches the kite, also a kite designed for single center line flag out is not designed to flag safely on a mini-5th line, you need to have the pulleys and bridle take up a particular load, so while it's safe to fly any brand mini-5th with any mini 5th kite and any single center flag bar with any single center flag kite be careful about mixing and matching those two types. In particular kites like the slingshot Z or ozone catalyst don't have pulleys so there's no possible way for them to safely flag on a mini-5th line liquid force kites even tho they are single line flag by contrast do seem to have enough bridle to fly them on a mini-5th, anyway this is probably more than you wanted to know