I have the 15x12 S.E.F. for minelab Explorer/Etrac. I was please with quality of coil but it is heavy. I use it mainly for wide open fields, etc where I want to cover alot of ground. Can't answer if it finds stuff deeper...In theory I guess it would as it is larger so the detection field is larger?? I did once dig a golf ball size piece of aluminum at about 30 inches with it though.
I still found small stuff with it though so no worries there.
I bought a 10x14 a couple years ago for an Ace 250 i used to own. great depth coil in open areas that are little to no trash. just bought a 10x12 butterfly coil for my M-6 a couple months ago but have been too busy lately to use it very much. seams too work really good on coins but not enough use yet to give a real good review.