I assume your question is the difference between the old heavy 8 and 10" coils that are usually refered to as "BBS Coils", versus the newer Tornado versions of both coils? If that's the case, I've only used the old heavy 8" BBS coil, which in reality is 7.25" in actual size like the newer Tornado version of it, so it's a very small coil. I found the old 8" BBS coils was fairly good in depth and stability in the short few times I used it land hunting (on an Excalibur a friend loaned me when I was in between machines), and I have read some say that they found the old 8" BBS coil was deeper and more stable for them than the old heavy BBS 10" coil, so they prefered it over that, or prefered the original 8" solid Coinsearch coil over either of the two sized BBS coils.
Now that the more modern/lighter versions of the 8 and 10" coils, called the Tornados, have come along with the advent of the Elite, seems both coils are talked very highly of. I can say the 10" Tornado is the best *stock* coil I ever owned on any detector I've owned or used IMO. Excellent depth, seperation, and stability, and the weight is pretty light too. In terms of an aftermarket coil, hands down the 12x10 for me steps all these aspects of performance up a notch for me, and even makes the tones and reaction time between targets more crisp and sharper to my ears.
The newer Tornados are also sometimes refered to as "Slimline" coils. Confusing to some, because that's what Minelab calles their FBS coils too. The 10" Slimline coils that I had on my Explorers are NOT the same coil as the 10" Tornado in terms of performance IMO. I never cared for those 10" coils on my Explorers at all. Worst stock coil I ever owned on a detector IMO. That's why many FBS guys have long since ditched those in favor of the 12x10 or the Pro Coil, and why you can't even seem to even be able to give those old 10" coils away for the FBS units. Minelab got it right with the 10" Tornado on the Elite/GT, and they also got it right with the 11" Pro Coil on the Etrac/SE Pro, but just the same many even seem to opt for the 12x10 over that coil on those units, believing depth, stability, and seperation has been enhanced even a bit more over the outstanding abilities of the Pro Coil. Not all though. Some have used the 12x10 and went back to the Pro Coil, but I'd say in general most seem to have opted for the 12x10 on those machines when they've compared both.
Either way, if you've got an Etrac or Explorer with a Pro Coil on it, it's one outstanding coil. And if you have a Sovereign with the 10" Tornado on it, once again Minelab hit a home run with that coil as well. The 8" Tornado is also said to be no slouch in depth and stability, and is said to really shine in unmasking in heavy trash or iron due to it's short 7.25" size that is a bit less than typical 8 or 9" small coils used in heavy trash that most machines come stock with. Having a somewhat smaller coil in that respect, yet from what I hear of it's depth potential still punching deeper than the 8 or even up to 10" coils I've used stock on all my prior non-Minelabs, is what has me very excited about the potential of this coil to bring "dead" sites back to life again. Minelab depth, yet with non-standard smaller coil size to see between the trash better in both the length and width wise aspects (compared to say a 8 or 9" concentric coil, or any 8 or 9" or bigger DD coil in terms of length wise respects).