I've had both, its basically frequency, coil, amount of discrimination. I didn't think there was that much difference performance wise in the two. Performance is obtained from understanding your equipment and how it works. if your looking for an upgrade go higher. (ATP) If you have a 250 add the DD coil. If you can afford the 350, I'd go that route. The DD coils makes a huge difference on the ACE units because there's no ground balance and the DD coil is better in bad ground.