Mark,my personal opinion is,and this is from a lot of hunting,that a coil should be chosen according to a few things. If your targets,say in an old ballfield,are known to be very deep(9"+), then you are going to have more success finding those deep coins with a coil that has a larger field. This would equate to a large coil,obviously. CAN a silver dime be found at 8-9" with an 8" coil? Yes. But the situation HAS to be perfect. A lot of targets are missed because of inaccurate coverage of the ground being searched,not because something changed since you've been there last. It's simply sloppy hunting,I do it myself sometimes. To reduce the chances of ground not being covered,a larger coil will help with that,as well as hit on the real deep oldies.
If you have a trashy place where the coins aren't necessarily all that deep(4-7") then one of the smaller coils works pretty well,it allows the machine to have to process less and allows the user to operate the machine with less discrimination,while at the same time not getting overwhelmed in the headphones with too many signals all clumped together.
Lastly,different coils handle EMI differently,some are terrible,others seem EMI resistant. The smoothest coil I've run on my Explorer was the stock 1050 coil it came with. It is deep as well,the deepest coin I found was a 10" silver Washington. Audio was a bit clipped and it had the Minelab "thunk" but it was repeatable and the numbers were close. That's a 10.5" coil.
What I use now on the Explorer are just 2...the Detech Ultimate 13" for wide open ball fields and the Sunray X-8" for everywhere else...when I use the Explorer2.
There are many different ones to choose from and performance seems to be similar amongst the similar sized coils. Just remember that in really trashy areas,you may not be able to see a 9" dime no matter what,so you're better off with a smaller one to get what you can. If the results don't suit you no matter the coil,then it's simply a matter of changing sites.
Good luck!