If you are using the stock coils they are mono coils (concentric) , vs a DD coil.
The benefit of using a Mono coil is that it goes deeper than a DD coil
of an equal size. However, Mono coils can be somewhat noisier in heavily mineralized ground.
A Mono coil is more sensitive to tiny gold and is easier to pinpoint with.
The benefit of using a DD coil is that if it is quieter and handles ground mineralization better. The penetration profile of a DD coil is such that you do not need to overlap your swings as much. But it does not go as deep as a Mono Coil
of equal size.
On say a 10" mono coil at 10 inches you search pattern may only be 2 inches wide, while on a 15 inch coil at 10 inches your search pattern is 15 inches wide.....
They say it is easier to pinpoint with a concentric coil, although personally I have no problem what so ever pinpointing with any of my DD coils. I have 3 of the 15 inch WOT coils, (one hardwired on my waterproofed Sovereign GT for use in the surf, one for my non waterproofed Sovereign GT, and one hard wired on to one of my 2 Excals and and I have one of the 15" x 18" SEF Butterfly coils that I use on my non waterproofed Sovereign GT, and I can pinpoint right on with all of them.... I personally much perfer the DD coils.
You would not get much more depth if any with the 11 inch DD Vs the 10 inch concentric, but you would get a lot more coverage since at 8-10 inches your search pattern is only about 2-3 inches wide....
Hope that helps some........