Perhaps the most noticeable difference between the Garrett Ace 250 and the Garrett Ace 350 metal detectors is the new 11 inch DD PRO-formance coil that comes stock on the Ace 350. Two of the traditional benefits of DD coils are increased target separation and better handling of ground mineralization.
In mineralized soils, the new Ace 350 coil does seem to improve stability, but only minimally. Depth wise, the Ace 350 is still comparable to the Ace 250 with a slight edge on depth. This is not surprising as metal detectors capable of handling heavy mineralization come at a much higher price tag. In mild soil conditions, the depth increase is more noticeable but with special attention being given to target depths. Many detectorists may not even notice the difference.
One of my reasons for trying the Ace 350 was to compare the signal response and target separation of the two metal detectors. While I like the ease of use and feature set on the Ace 250 for beginner detectorists, a noticeable drawback to the Ace 250 is the somewhat slow signal response that occurs when passing over a metal target. The audio response kicks in after the coil has passed the target. This can make pinpointing without using the pinpoint feature more difficult than it should be for first time users of this metal detector. Unfortunately, the Ace 350 also suffers from this delay in response.
As for the target separation, I have noticed an improvement in target separation. However, the delayed target response limits the benefits of the narrow hot strip of a DD coil. Additionally, the target id seems a little more jumpy, which is common when comparing a DD coil to a concentric coil.
Another difference between the Ace 250 and Ace 350 is the operating frequency. The Ace 350 runs a little higher at 8.25 kHz as compared to 6.5 kHz which should make the Ace 350 more sensitive to smaller low conductive targets like foil, nickels, gold, and pull tabs. Realistically, the frequency is still too low to make any huge claims about the Ace 350 being noticeably better on finding small gold. It is, after all, an entry level multi-purpose metal detector.
While both metal detectors utilize 12 target id cursor segments, the Ace 350 incorporates four iron discrimination segments instead of two as found on the Ace 250. This may be beneficial to relic hunters interested in finding some but not all iron targets.
So, how does the Ace 350 compare to the Ace 250 metal detector? Overall, the Garrett Ace 350 does make some improvements over the Ace 250, but are the slight benefits worth paying for. That is really a subjective and individual call, but I personally do not see the Ace 350 as offering much enough of an upgrade to justify they added cost. For an entry level metal detector with a digital display and multiple search features, the Garrett Ace 250 provides a better cost to performance value. (You might be interested in my Garrett Ace 250 tips page)