Well, I'm biased beacause I also own an ACE250. I just took a look at the Prizm IV on the Whites website, and the 2 machines seem to be about the same in terms of features. They both offer Target ID, Depth Readout, Pinpoint, Automatic balancing and sensitivity control, multi-tone ID, and the screens appear to show the exact same info just with different layouts (although the readout on the Prizm seems a bit larger and easier to read). The only difference seems to be in the coils, with the Prizm offering an 8" round coil, while the ACE250 has a 6.5" x 9" concentric coil, which to me is 6 of one and a half dozen of another. I've hunted with both a round coil and concentric, and there is really no difference at all. The only other difference appears to be PRICE. The Prizm IV retails for $399.95, while the ACE250 retails for $199.95. I personally am VERY pleased with the ACE250, it is light years ahead of my old Bounty Hunter. When I was considering the ACE250, I compared it to the Prizm II, which runs around the same pricewise, and I chose the ACE250 bcause it had depth readout and multi-tone ID, which the Prizm II did not. I think the comparison between these 2 machines comes down to price. Both offer the same exact features, but the ACE250 is half the price. And with so many users here echoing what I think of the machine (as a great detector for the price), I don't think you can go wrong with the ACE250. Just my opinion though. Good luck, whichever machine you choose will be a good, quaility machine that will give you years of enjoyment!