general or sens. 15x12 commander

Hi all!
I was wondering what the difference is in performance between normal mode and sens extra when using a coil like 15x12 mono on a 4000? And also, and what conditions suit each mode the best? What type of ground is classed as highly mineralised? I usually detect heaps of hot rocks but am unsure if i should be running sens. smooth, or dig all the hotrocks?
The area is fairly shallow mainly up to a gram nuggets. It is fairly noisy at times and a flight path is overhead.
Any help on setting up the 15x12 on the 4000 would be great!
I have been using a coiltek 10x5 mono around the old diggings along the creek but is slow going and really sens. Made me think about putting on the 15x12 and going over the paddock?
Cheers!

Goltzy,if your prospecting over hot ground,I would recommend staying in General/Sensitive at the front end and in sensitive smooth on the rear.That's if you seek to keep your sanity with both ground noise and hot rocks (highly mineralized ground).Sensitive Extra to be used over ground that is low in mineralization.It provides better signal response on small deep targets.Normal mode will provide the greatest of coil field depth on all size targets, but the ground has to be basically benign(neutral),or very low in mineralization to get the coil quiet and stable enough for use.
Settings Front Cap;General,Sensitive,Fixed,Mono,Threshold barely audible,with tone to suit. Rear End; Signal 15,volume 25 or to suit.Audio tone 50 or to suit you,tracking slow,Sensitive Smooth,Ground Balance type General.Iron reject Off. Motion swing speed Slow. Response Normal. Gain,start with FP 8 + or -2,higher or lower as required,depending on degree of ground mineralization.

Cheers Kon. 

Cheers for the reply Kon! Will give your suggestion a try on the weekend!