Can't go wrong with the ACE detector series, an Id like the 400 very much. As noted above, some improvements from Ace 300 to Ace 400, but if you're looking for an upgrade, consider the AT Pro (My middle sister had a choice between Ace 400 at about $350 and AT Pro at about $550. and she chose the AT Pro). I believe it would also be considered an upgrade from the AT Pro to the Equinox 600 (about $700). And some improvement, but not an upgrade from the 600 to the 800. I just took delivery of the 800. Of course, if money is not a concern we all would probably purchase the most expensive machines, but when money is tight, as for most of us, we have to look at practical choices. Personally, I think the Ace 300 is a solid machine and if you're looking for a true upgrade, save your finds and seriously consider the AT Pro for a true upgrade. B]