I will apologize in advance for my convoluted answer. I have the Super 12, 9.5 stock, 6x10DD, and 4x6DD. I use different coils depending on areas, conditions and levels of trash.
The 12 is typically reserved for relic hunting in areas where there is very little trash. I use this coil less than all the others. Not because I don't like it. It's a fantastic coil. Still, in any area with even moderate trash, target masking and multiple targets under coil can give the detector fits. You'll get tired moving lots of dirt. Invest in a quality shovel or this coil's depth will beat you to death.
I use the 9.5 coil in light trash areas or tot lots that appear to be cleaned out. It will pull stuff that others miss. Because of it's sensitivity, you can't work to close to playground equipment. Still, I love to run it when I get to a tot lot or area that appears to be picked clean. Run it at +1 or +2, whatever you can get away with. If there is anything there that was missed, the stock coil will find it.
I use the 6x10DD in areas of all but heavy trash. Parks, tot lots or any place that I need target sep. but need to cover a fair amount of ground. It chatters a bit more than the other coils. I tend to run mine in relic mode, pushing a bit on sens, and just dig anything that even remotely resembles a high tone. Good depth, fair area coverage, can work in minerals and iron, and can find the small stuff.
The 4x6DD I use regularly in tot lots. I can ID those targets that are all but pushed up against play equipment. It's also the king of working along chain link fences. It's only down fall is that area coverage is slim to none. You have to be very aware of your swing and constantly overlap swing and stroke, or you will miss stuff.
My friend runs the 5.3 on his M6 and swears by it. Based on his finds, it serves him really well. I don't know how it does in heavy minerals/iron. We have a spot we are hitting tonight that had a foundry nearby. I'll comment on his success tomorrow.
I would either purchase the 4x6 or 6x10 as your 1st alternate coil. In fact I would lean toward the 4x6. It will work in heavy minerals, iron rich environments, and get you very close to equipment to find targets that others missed because they were using a coil that picked up signal from equipment, masking the target.
Some will swear that 6x10 should be your second choice. While I agree that I prefer to run it, there have still been more than a few places that were so trashy that even it had a hard time and left me pining for the 4x6.
Based on everything I have read and heard, in addition to speaking with 2 people that attempted to run them, I would avoid the 14.
I hope this helps.