I have the NEL Thunder. I chose it over the NEL Storm for a couple of reasons:
1) The coil is lighter, resulting in more swing time.
2) The coil is longer, resulting in more coverage per sweep
Both are DD coils, and while the Storm could go deeper, I predominantly hunt in Schools and parks, in soil less than 8 inches (and that is rare to go that deep.). Most parks and schools here are less than 30 years old, over half less than 15.
I upgraded to the NEL Hunter (halfway between Stock and Thunder), 2 years ago, and instantly started finding more items. I ran the equipment over the same places I'd been hunting for a year, and KNOW I missed gold with the stock equipment, because it couldn't pick off the gold from the iron it was literally sitting on, or next to (rebar, in the two cases). I had a strong feeling there was gold in that area, but I couldn't ever get anything other than iron signals with the stock coil. The NEL Hunter coil picked it right off. I've found excellent results from the NEL series DD coils in target separation.
Some folks absolutely adore and love the 5x8 coils, but I've found the coverage to be so superior with a 14.5" coil that I can literally eat up an entire soccer field in a couple hours of hunting. At best, I could do half with the stock. This translates to more finds (IN MY ENVIRONMENTS). A 5x8 would be exceptional in heavily trashy areas, but I've found my Thunder is more than capable of picking out gold signals (or rather, my ears are WITH the Thunder), from those same areas. I've pulled 3 golds from one of the most heavily trashed parks in Boise Idaho (Ann Morrison), with them in the middle of trashy swings (think 5-12 signals per swing). The key is hearing the sounds, and I couldn't ever do that with the Stock coil, the DD NEL series just seems to pick them out better. Sure it's a pain in the rear to isolate objects with a signal blade of 14.5 inches, but it can be done, playing the angles.
But when you're in a wide open space and signals are far apart, there is NOTHING better than a long DD coil to eat up the turf. That's where it really shines.
Cheers!
Skippy