Thank you to the OP for this thread, as I also am considering a PI detector in the near future. There hasn't been much discussion in this thread thus far about the ATX, and I think the Garrett has gotten a bit of an undeserved "bum rap" as a nugget detector. Yeah, it's a bit heavy, and not as ergonomic as others, but for the price, it does an admirable job. I think if you put the 8" monoloop on it, it will pick up the same crumbs as the SDC. The standard ATX coil won't get the crumbs, but it will reach deeper on larger gold targets than the standard SDC coil. As with all detectors, they all have their compromises. If you want to add a Coiltek to the SDC, you've just increased the price point by another several hundred dollars. Worth it against the ATX? (The same could be said for purchasing the 8" monoloop for the ATX. Are crumbs worth the investment?)
One very significant advantage the ATX has over Minelabs, is either the proprietary battery Minelab puts in the GPX and GPZ series, or the rechargeables in the SDC. This advantage is becoming less of an issue these days with the advent of portable battery power-supplies, but if you plan to be in the bush for several days, and don't want to take the risk of running down your car battery to recharge detector batteries, the ATX also can use alkalines. If you spring for the GPX or GPZ, and are going to pound an area for several days, you really need more than one of these expensive battery units...another several hundred dollars.
I think the best way to measure one versus the other is budget. The two detectors do not perform equally, out of the box. If you have the budget for the SDC, and bedrock is fairly shallow in the places you plan to detect, then I would lean towards the SDC. If the SDC is a bit of a reach financially, and the areas you wish to search might have some nuggets beyond the 12" (or so) reasonable limit of the SDC (yes, I know it can pick up a tin can at two feet), then I would go with the ATX.
I have heard that there might be an issue with Chinese counterfeits of Minelabs floating around. If you purchase an SDC from a genuine Minelab dealer, you'll pay an additional $500 for a machine. (That puts it $1,700 more than a standard ATX.) Peace of mind? Lots of eBay sellers pricing their SDC machines at $3,299, but what about warranty, etc.? No worries about the ATX, as Garrett is an American company.
An issue with each machine that others have experienced. The SDC apparently has a "known" issue with the batteries making poor contact with the electrodes, occasionally causing erratic behavior when bumped. There is a fix for this, but I can't remember it at the moment. (Was it putting some sort of shim in the battery compartment?) The ATX can false when the coil is bumped against terrain, and apparently this is caused by the cable bouncing inside the stem/arm.
One other consideration, that both detectors can claim, is their ease of use. The SDC is perhaps a little easier, perhaps, but they are both good candidates for folks who don't have a lot of time to invest understanding tunings, different settings, etc.
I attempted to use a very good VLF machine in northern California, in the Klamath region, and discovered that the mineralization present made using it in all-metal impossible. The only way I could get consistent performance was to use the discriminate mode. I wonder now what kind of luck I might have had with a PI.
Just my two cents...