SM6K covered a lot of the points very well. To cap off on that, when the DD coils first became available, I watched a number of videos of them in action, particularly listening to the tonal response of the targets, including the width of the target. I also watched them for audio response in pinpoint. I was looking to see if they gave off useful audio information. They didn't, so I never bought one. I do have the 5x10 DD coil on my 1500. That is a whole different ball game. On the 1500, coin sized targets give a distinct slightly narrower tone, on coins depending, on the denomination. I'm an Aussie detectorist, so I'm looking for our $1 and $2 coins, which hit on the same notch, as a US zinc penny. These coins are a common find, but you are dealing with aluminium screw caps that hit in this area. Unlike the coins, they give off a full tone.
As far as the metal twist tops go, they also hit in close to this area, and yes, they can sound good and fool you, but speaking from experience (not with a DD coil on an Ace though) I find that they usually give themselves away, by 1, sounding inconsistent and 2, by have the visual ID bounce around and not properly lock into a notch. Mostly you can tell by tone though. As an example, I hit a particular location a few years ago (posted about on the forum). This spot was utterly infested with steel twist tops, from memory, I was getting a minimum of at least one twist top hit every 3 sweeps of the coil and at times up to 4 to 6 hits each sweep. Yet for all these hits, I would have pulled out less than 20 twist tops, but pulled up $118! I was there for an all day hunt.
Getting back to the Ace, the one and only advantage, that I can concede, is that it will give you better target separation than the concentric coil. That said, as mentioned earlier in the post, If I swing an Ace again, it's with the concentric coils only (including the sniper) as they have properties, that are almost the exclusive domain of the Ace detectors. The Sniper coils is almost unmatched in its ability to hunt around metal play equipment if you hold in the pinpoint button and tilt the coil ever so slightly away from the metal pole you are hunting around. Also you can hunt under metal poles with the coil touching them (only in pinpoint mode though).
Mick Evans.