I have the following coils for my ETRAC: WOT, stock, X-5, SEF 8x6, SEF 15X12, ML 8, and the Joey. In extememly high trash areas, such as a scraped lot, I will use the X-5, SEF 8x6, or the Joey. In sparse target conditions, it is the stock, the SEF 15X12 or the WOT. for the depth. In normal conditions, I will use the stock, or the SEF 15X12.
There are times however, when I use the Joey coil. That footprint is large (11 inches long), by not so wide (5 inches width). I get excellent separation with that coil under a lot of differing conditions.
It has been around my arsenal for many years, starting with my EX II. I still use it at times. With TTF, it can be a killer of a coil. Here is something I would suggest for anyone......
Put aside a bit of money, and then, every 3 or 4 months, buy a coil and stem for your detector. I have a lower stem for each of my coils, and OMG, does it EVER save time of changeover PLUS, if I happen to BREAK a lower stem, I am not stuck in the field 100 miles from home without a coil/stem combination. If you have ever had that happen, you understand the wisdom of having a second stem.
As far as size, I would suggest that as a second coil, go for the X-5. or the 4X7. Those small footprints will excel in high trash. Then, go Large, say te 15x12 or WO. Finally, don't pass a joey by if you see one. I would put it up there above my SEF 8x6, or any other 6 or 8 inch coil in terms of performance.
But I would start with something really small, because in high trash conditions, which other detectorists will have given up, you will find a producer of coins in the smallest coils.
HH
Dennis