Experience has shown that concentric coils generally give more accurate ID of targets. All DD coils really hit hard on bottle caps and usually ID them as a good target. Concentric coils tend to read them more often as a bottle cap or junk. The verdict is still out on which coil is better at target separation. A small concentric coil (4.5" Sniper) will get closer to a chain link fence, re-bar and play equipment then a small DD coil. Choose your coils wisely, as they both have there strengths and weaknesses over each other.
Some useful coil information - http://www.garrett.com/hobbysite/hbby_searchcoil_tech_sheet.aspx
Some useful coil information - http://www.garrett.com/hobbysite/hbby_searchcoil_tech_sheet.aspx
