From my perspective...if I were given one...I'd take the GP Extreme...if I were to buy one..I'd take the Goldquest SS <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="

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Now that's just from the point of view of bang for the buck. From the reports that I've read, it's hard to beat the value of the Goldquest SS for what it can deliver in "most" areas, but the GP Extreme is a very complex(and heavy)detector that many consider to be the most sensitive and deepest seeking gold nugget PI machine on the market...and it has a price tag to match those credentials as well.
I think it would be great if you could try both in your area before purchasing one. You may find other things besides depth of the machines to be a determining factor in which detector you finally decide on..such as ease of use, weight,balance,etc.
The GP Extreme was developed in Australia to handle the worst conditions possible while still being sensitive to reasonably small nuggets and it sounds like it has many adjustments and features which help it detect well in almost every situation.
The Goldquest on the other hand seems to me to cater more towards those that want a versatile,light, deep seeking PI detector that is easy to use with a minimum of adjustments.
They are both deep seeking PI detectors but they are designed very differently and each suits different personalities in my opinion.
I think it's kind of like the X5 vs MXT debates when they first came out...each camp had it's supporters and some favored the target ID style MXT while others favored the more basic but lighter and well balanced X5. They both detect deep but each has other features that in the end make a bigger difference than an extra inch in depth on the occasional target.
All just my opinion as I sort through all the different machines on the market...it's finding the detector that matches the personality,experience and patience level of the detectorist that matters the most.