This is a tough decision.....I truly love my 5 x 8 coil DD. I use it all the time in the woods and it has rewarded me well. But, I also like using the 9 x 12 concentric coil. It gives great ID at better depth, is sensitive to tiny objects, ID's better then a DD coil and ID's most bottle caps as junk. Considering my favorite targets are rings, I would pick the 9 x 12 coil, as that is the coil I use hunting in freshwater and open spaces. The 5 x 8 DD coil would definitely be my second choice.
I agree with tabman. I prefer the NEL SHARPSHOOTER over the stock coil and I don't think I lost any depth compared to stock. I run it at max sensitivity almost anywhere I go. Those are the only 2 coils i have so I cant say the other coils wouldn't be better or worse. It all depends on which kind of areas you hunt and how deep you want to dig when deciding on which coil you want to buy.
Man, I'm really solidly 50/50 on the 9x12 and 5x8 coils... but if I could have only one, it would be the 5x8. I'd just be really slow covering ground in more open areas like fields and yards... have to stick more to cellar holes and old home sites where the slower going is a real benefit.
Garrett At Pro, 8 x11" and Nel Storm coils
Garrett Propointer er, Pro Pointer AT
White's V3i Standard, 10" DD, 13" Ultimate and 4 x6" sniper, 6 x10 coils, Drect 12 x 15 coil
Whites MX Sport With Detec
I love the 5x8 as well. If I were going to be digging a lot of deep targets though I would go with my Nel Storm. With that combo you can literally detect almost anywhere in any conditions.