I have 3 different coils

i'd use the big 12" coil in more open cleaner areas (fields, cleaner big yards)it covers more area and will get more depth
the smaller 9" coil if its the "imaging coil" is the stock coil it came with and is a great all around coil.
the DD is good for highly minerilized areas, and also isn't too bad at seperating targets in trashy areas, normally in highly trashy areas a 4-6" sniper coil is ideal
the 10x14 DD won't give you the "size imaging" on your display

but to sum it up i'd mainly use the 9" stock imaging coil

I also have the GTI2500 I use a 5 X 7” DD coil most of the time. Even though it’s not an imaging coil I can tell how big the items are that I’m scanning when pin pointing them. I just received the 8.5 X 11” DD on Monday and use it today for the first time, I had a hard time pin pointing with it at first but now I think I have the hang of this coil. I did find some older deeper coins today so maybe this coil helped a little. The coils I have and use are…
4.5” DD – Good for trashy areas or prospecting for Gold (it will pick up a 2.4 grain gold flake at 2” none of the other coils can see this small flake of gold at all.
5X7” DD – I use this coil most of the time except when prospecting. Light and easy to swing, good in trashy areas
8.5 X 11” DD – New – Not so good in trashing areas goes deep
9.5” Imaging Stock Coil – I don’t use this coil to much anymore. I can get the same information from the small non-imaging DD coils when I pin point a target. After using the 5X7 coil for a while the 9.5” coil is very heavy to swing. I live in Colorado and our ground is highly mineralized, I would get a lot of false readings with the stock coil, even after turning the sensitivity way down. This is the main reason that I don’t use this coil much anymore.