The target signal should be at the strongest in the centre of the coil, just as it is with the larger 9" coil. The 6" is mainly designed for working junky areas, whereas the 9" should give a depth advatage as well a bit more ground coverage than the smaller coil. The downside of the larger coil in junky areas is the lesser ability to seperate out targets, as it may in some circumstances end up with several targets being picked up at the same time under the coil. The smaller coil will allow you to pick up good targets a lot closer to nearby junk, and allow a more accurate ID.
I wouldn't think the tones would be any different between both of them.
Here's a diagram showing the difference in detection field between a double D coil, large concentric, and small concentric. The concentrics offer up a cone shaped detection field and more accurate target ID's, whereas the DD will offer better handling of mineralisation, and will be easier to seperate out targets with its blade shaped detection field (down the centre axis of the coil). Hope that helps a bit.
https://www.prospectingaustralia.com/forum/img/member-images/1695/1468746695_images-3.jpg