Nero_Design had a picture of the detection field of the 6 HF and 5x10 HF DD's. It shows the 6 HF's field is a much narrower knife blade(also better for trash separation), while the 5x10 HF has a wider field along the center. The 6 HF will find a bit smaller gold and is a specialty coil but it is not for general field detecting in the goldfields due to noisier in very high mineralization and less coverage. The 5x10 HF is quieter in very high mineralization with more coverage. Both do not handle some hotrocks well just like all other VLF's also. I just got this coil recently and have not tried it yet as I wonder if the round 10.5 HF DD handles hot rocks and mineralization better? But yes it will not find as small of gold as the other two coils. If you want the very best hotrock rejection use the 70(5) in C & T Mode or use a Minelab SD/GP(X) machine.
"...the X-70 picked up hotrocks( -8 and +48 ) in certain areas and is a problem sometimes. I have heard the new Off-Set Tracking on the X-705 will help with hotrocks. Also the 5x10" DD HF does a bit better on hotrocks than the 6" DD HF."
ALSO, I knew this before that all BBS and FBS machines are not good on very small gold items(thus not recommended for prospecting for small gold nuggets), but this is still interesting what Sky1 said very recently on the Safari/Quattro Forum, and I added in (brackets):
http://www.findmall.com/read.php?3,1137338
Re: CONFUSED -Safari vs Sovereign GT
"I don't know about the sovereign(BBS) but my Safari(FBS) is not that exceptional on (gold)jewelry. It will NOT pick up small studded diamond ear rings, nor will it detect thin white gold, it will not detect small nuggets either."