If I had it to do over again I think I would have 3 coils. Whatever stock coil that came on the detector and two others. When I bought mine, the gold pack coil was not available so I have the 9" concentric which I haven't had on the detector in nearly two years. I always recommend to anyone interested in buying a 705 to get the gold pack coil just because the HF coils are very forgiving (more sensitive may be a better choice of words) and have what appears to be faster recovery time when scanning over items.
I'd have a hard time picking between the two 10.5" coils. Lately, I've been favoring the MF but in years past I've found most of my stuff with the HF. I've never found anything notable with the 6" MF coil but the HF coil is a must have for trashy or tight locations. Don't get me wrong, I use them all throughout the year (except the 9" concentric) but 90% of the time I'm using one of the two 10.5" coils and then occasionally the 6" DD HF coil.
We've been working a local 1800's property and I've used every coil but the 9" concentric. The next decent day we have, I'm going to give it a shot and see if I come up with anything that the the other coils haven't recovered.
if I had to sell off one of the coils the 6" MF would be the first to go. We sometimes have 3 or 4 of us out detecting with 705's so although I typically only use two coils on an outing it's nice that I'm able to share any of the other coils if someone wants to give them a go. When I first bought my detector I was really just trying to experiment by buying so many coils but I eventually found out that most of them have their place depending on where we are detecting.
