Metal detector choices are a personal thing. Some people get blind-sided by hype and never will believe many things even if they are staring at the truth right in front of their face. Some people are so brand-loyal that they won't even look at another brand even if one of those detectors produces better results. I know people who say they are Democrats when they are actually Republicans. I knew a man who found more goodies with a metal detector than any other person I've ever met. He used an old Compass from the early 80's, yet tried many others. He found guns, knives, lots of knives, rings, jewelry, relics, gold, silver, old coins and many other nice things, EVERY DAY HE USED IT HE FOUND AT LEAST ONE NICE PRIZE. He owned a Tesoro Silver Sabre, but only kept it as a backup. He never did use discrimination, all-metal was his game. He actually earned a meager living with his searches.
The best gauge for sensitivity settings is to turn it up until the detector begins to chatter, then turn it down 1/2 to 1 turn. An air test is what to use because then the test variables are constant, while in ground are not. We old guys know how to use sensitivity up at a higher setting, but not all of us do, the sounds are way to complex for most people's ability to discern them all.
The reason that there are so many Aces on eBay and in pawn shops is because some people get tired of the bong, bong noise, but especially because the Aces produce inferior depth results in higher Fe soils. I can show a video of their weaker but not devastating results, but people can do it their own self by going to Youtube. Here in the western USA Garretts are seldom seen and when they are, they are usually found in a pawn shop. Many Minelabs are found there too. The only Minelabs and/or Garretts I've seen here in the last 10 years were mine, plus two of each in a retail detector shop run by my friends. The reason is that Whites and FT (Fisher and Teknetics) simply work better here and produce overall better results in our harsh soil. Nobody sells something that will come back to haunt him and then have to give them their money back, that would be foolish. I don't have any opinions about this, it just is what it is.
The ATP was an attempt at competing against the Whites MXT and the Fisher GBP and similar. Many people are getting rid of their ATP's because they discovered that they could have gotten a better deal from another brand, namely Whites and First Texas, so they dumped the ATP ASAP. Garrett still uses early 80's technology on their single-frequency, delayed response machines. They are very similar in operation to the old Bounty Hunter Red Baron and the Whites 6,000's. That is why the boing sound is delayed and sounds familiar, it's old technology improved to some degree.
Tesoro uses less complicated but more reliable (older) technology. This is why Tesoros are such a good deal for the money, simple, but seldom are depth demons. Their discrimantion is very commendable though, and so is recovery and retune speed.
First Texas (FT, Fisher, Teknetics) is ahead of the game right now. Whites too, but they are not going very far, we have reached a plateau in metal detector technology. We can't crank in more power, partly because of too limited returns, and partly becaause it could interfere with police, ambulance, and medical procedures operations. All we can do now is to add more toys, perhaps a tax program, or maybe a weather alert, a magazine rack, or recipe or two for making delicious fudge to our hobby list? That way we would have more toys on our toy detectors and everybody smiles, and everybody wins.
The bottom line: NEVER buy before you try. Don't listen to hype, it's probably just more BS. "If it walks like a duck"... And go with a successful person and follow his or her advise and their performance, it is the best teacher. Don't rely on shop owners or WalMart for advice, unless they are active metal detector enthusiasts, and that too is not guaranteed good advice either. Lastly, JOIN A CLUB! You get to compare detectors in the flesh, instead of being reduced to just reading stuff like what I just wrote. You all have a nice day, I'm going out to find a new hot looking woman somewhere, and yes at my age of 70.